drm/amdgpu: remove non-sense NULL ptr check

It's a bug having a dead pointer in the IDR, silently returning
is the worst we can do.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>