drm/amdgpu: change clock gating mode for uvd_v4.

use sw cg when decode. and hw cg when idle.

fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=99313
https://bugzilla.kernel.org/show_bug.cgi?id=192161



Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Ack-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>