drm/amdgpu: drop the extra sign extension

amdgpu_bo_gpu_offset_no_check() already calls
amdgpu_gmc_sign_extend() so no need to call it twice.

Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>