drm/amd/display: refactor init_hw to isolate pipe related init

[Why]
Pipe related init is possible to optimized if we know what we
intend to program, and if we can determine it matches what is
already programmed for the pipe.

[How]
First step is to isolate the pipe related init code

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>