drm/amd/display: Split program front end part that occur outside lock

[Why]
Eventually want to lock at a higher level in stack.
To do this, we need to be able to isolate the parts that need to be done
after pipe unlock.

[How]
Split out programming that is done post unlock.

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>