drm/amd/display: refactor out programming of vupdate interrupt

[Why]
More clearly isolate the code that is involved in programming of
vupdate interrupt

[How]
Add function for programming of vupdate interrupt.
Call it after timing is programmed.

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>
Acked-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>