drm/amd/display: isolate global double buffer lock programming

[why]
Global optic double buffer lock is currently disabled due to
incorrect programming sequence that affects non global lock.

[how]
Isolate global lock programming sequence out of non global lock
programming sequence, so it can be enabled without affecting
non global lock.

Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>