drm/amd/display: Intermittent DCN2 pipe hang on mode change

[Why]
GSL is being used to synchronize pipes when vsync is off but
on transition to vsync on during a mode change GSL is not
being reset correctly.

[How]
Disable GSL on any plane that is disabled.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>