drm/amdgpu/gfx11: remove some register fields that no longer exist

Some copy paste leftovers for older asics.  They were protected
by __BIG_ENDIAN, so we didn't notice them initially.

Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>