drm/amdgpu: remove unused variable

Remove set but unused variable.
warning: variable 'umc_reg_offset' set but not used

Signed-off-by: mziya <Mohammadzafar.ziya@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>