drm/radeon: also init GEM funcs in radeon_gem_prime_import_sg_table

Otherwise we will run into a NULL ptr deref.

Signed-off-by: Christian König <christian.koenig@amd.com>
Bug: https://bugzilla.kernel.org/show_bug.cgi?id=212137


Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org # 5.11.x