drm/amdkfd: Import DMABufs for interop through DRM

Use drm_gem_prime_fd_to_handle to import DMABufs for interop. This
ensures that a GEM handle is created on import and that obj->dma_buf
will be set and remain set as long as the object is imported into KFD.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Ramesh Errabolu <Ramesh.Errabolu@amd.com>
Reviewed-by: Xiaogang.Chen <Xiaogang.Chen@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>