drm/radeon/cik: Read back SDMA WPTR register after writing it

For symmetry with other *_set_wptr hooks.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>