drm/amdgpu: A workaround for JPEG_v4_0_3 ring test fail

The jpeg_v4_0_3 jpeg_pitch register uses UVD_JRBC_SCRATCH0. It needs to
move WREG() to after jpeg_start.
Switch to a posted register write when doing the ring test to make sure
the register write lands before we test the result.

Signed-off-by: Sonny Jiang <sonny.jiang@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>