drm/amd/display: Re-initialize viewport after pipe merge

[Why]
Pipes get merged in preparation for SubVP but if they don't get used, and
are in ODM or some other multi pipe config, it would calculate the
voltage level with a viewport of just one pipe from when they were split
resulting in too low of a voltage level.

[How]
Made it so that the viewport and other timing settings get rebuilt and re-
initialized after the pipe merge, before calculating the voltage level so it
would calculate it correctly.

Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Ethan Wellenreiter <Ethan.Wellenreiter@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>