drm/amdgpu: Make umc_v8_10_convert_error_address static and remove unused variable

Fixes following warnings:
warning: no previous prototype for 'umc_v8_10_convert_error_address'
warning: variable 'channel_index' set but not used

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Candice Li <candice.li@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>