drm/amdgpu: If amdgpu_ib_schedule fails return back the error.

Use ERR_PTR to return back the error happened during amdgpu_ib_schedule.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>