drm/amdgpu: fix unchecked return value warning for amdgpu_gfx

This resolves the unchecded return value warning reported by Coverity.

Signed-off-by: Tim Huang <tim.huang@amd.com>
Reviewed-by: Jesse Zhang <jesse.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>