drm/amdgpu: remove useless BUG_ONs

Dereferencing NULL pointers will cause a BUG anyway. No need to do
an explicit check.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>