drm/amd/display: add preferred pipe split logic

drm/amd/display: add preferred pipe split logic

[why]
existing logic finds "first free pipe from 5 -> 0" to split
this will cause certain sequences to require DC to move
an MPCC from one tree to another, which is unsupported
this leads to blackscreen

to mitigate this problem, we will always try to acquire the
"preferred" pipe, and each pipe has a unique preferred pipe
this means we avoid most of the scenarios where
pipe splitting leads to moving MPCC from one tree
to another

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