drm/radeon: rework UVD writeback & [rw]ptr handling

The hardware just doesn't support this correctly.
Disable it before we accidentally write anywhere we shouldn't.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/cik.c
Simple merge

diff --cc drivers/gpu/drm/radeon/evergreen.c
Simple merge

diff --cc drivers/gpu/drm/radeon/ni.c
Simple merge

diff --cc drivers/gpu/drm/radeon/r600.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon.h
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_asic.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_asic.h
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_ring.c
Simple merge

diff --cc drivers/gpu/drm/radeon/rv770.c
Simple merge

diff --cc drivers/gpu/drm/radeon/si.c
Simple merge