drm/amd/display: Indicate dsc updates explicitly

[Why]
DSC updates only set type to FULL UPDATE, but doesn't
flag the change

[How]
Add DSC flag update flag

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>