drm/amdgpu: fix a build warning by a typo in amdgpu_gfx.c

This should be a typo when intruducing multi-xx support.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Cc: Le Ma <le.ma@amd.com>
Reviewed-by: Le Ma <le.ma@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>