drm/amdgpu: remove unused variable

building with gcc and W=1 reports
drivers/gpu/drm/amd/amdgpu/gfx_v10_0.c:7292:11: error:
unused variable 'tmp' [-Werror=unused-variable]
  uint32_t tmp;
           ^~~

tmp is not used so remove it.

Signed-off-by: bobzhou <bob.zhou@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>