drm/radeon/tn/si: enable/disable vce cg when encoding v2

Some of the vce clocks are automatic, others need to
be manually enabled.  For ease, just disable cg when
vce is active.

v2: rebased, call vce_v1_0_enable_mgcg directly

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