nvdec: fix multiple call to cuInit

/!\ Disclamer: because of the usage of vlc_once_t, I'm not sure whether the device pointer used for initialization needs to be atomic or not. Feel free to indicate whether this incorrect, I haven't checked yet.

Each cuInit is leaking memory, and cuInit is supposed to be called only once. Ensure it through a vlc_once_t wrapper, but since the function table needs to be loaded in order to call into cuda functions, forward some available device for the initialization.

Below, there is two leaks because of the double decoder device allocation in debug mode. The first leak is leaked once, and is still there. The second and third leaks are now reduced to a single one.

Direct leak of 65536 byte(s) in 1 object(s) allocated from:
    #0 0x7febb6bf3652 in __interceptor_realloc /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:164
    #1 0x7feb8b56828f  (<unknown module>)

Direct leak of 56 byte(s) in 1 object(s) allocated from:
    #0 0x7febb6bf3459 in __interceptor_calloc /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:154
    #1 0x7feb89dc586a  (/home/alexandre/workspace/videolabs/vlc-meson/build-native/modules/.libs/libskins2_plugin.so+0x2f1086a)
    #2 0x7febb5f195c1 in decoder_device_Open ../../src/input/decoder_helpers.c:174
    #3 0x7febb5e67106 in vlc_module_load ../../src/modules/modules.c:243
    #4 0x7febb5f196e8 in vlc_decoder_device_Create ../../src/input/decoder_helpers.c:188
    #5 0x7febb60a7435 in vout_GetDevice ../../src/video_output/video_output.c:2244
    #6 0x7febb5ef7f26 in ModuleThread_GetDecoderDevice ../../src/input/decoder.c:608
    #7 0x7feba02ee139 in decoder_GetDecoderDevice ../../include/vlc_codec.h:304
    #8 0x7feba02fb51e in lavc_UpdateVideoFormat ../../modules/codec/avcodec/video.c:286
    #9 0x7feba0313ff7 in ffmpeg_GetFormat ../../modules/codec/avcodec/video.c:1682
    #10 0x7feb9f0bfe12  (/usr/lib/libavcodec.so.58+0x29ae12)

Direct leak of 56 byte(s) in 1 object(s) allocated from:
    #0 0x7febb6bf3459 in __interceptor_calloc /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:154
    #1 0x7feb8b5d386a  (<unknown module>)
    #2 0x7febb5f195c1 in decoder_device_Open ../../src/input/decoder_helpers.c:174
    #3 0x7febb5e67106 in vlc_module_load ../../src/modules/modules.c:243
    #4 0x7febb5f196e8 in vlc_decoder_device_Create ../../src/input/decoder_helpers.c:188
    #5 0x7febb60a7435 in vout_GetDevice ../../src/video_output/video_output.c:2244
    #6 0x7febb5ef7f26 in ModuleThread_GetDecoderDevice ../../src/input/decoder.c:608
    #7 0x7feba02ee139 in decoder_GetDecoderDevice ../../include/vlc_codec.h:304
    #8 0x7feba02fb51e in lavc_UpdateVideoFormat ../../modules/codec/avcodec/video.c:286
    #9 0x7feba0313ff7 in ffmpeg_GetFormat ../../modules/codec/avcodec/video.c:1682
    #10 0x7feb9f0bfe12  (/usr/lib/libavcodec.so.58+0x29ae12)