vulkan/gpu_buf: don't assume host ptr imports are mappable
The logic in vk_buf_flush treates buffers imported from
PL_HANDLE_HOST_PTR identically to host_mapped buffers, but this
assumption is only true if we assume that the host pointers were
imported to a host-visible memory type. This is not always the case.
Relax the restriction to only require invalidating mapped memory if the memory is actually mapped.