drm/radeon: rework pll selection (v3)

commit 985f61f7ee647ad570c05eab0b74915da2ac8e19 upstream.

For DP we can use the same PPLL for all active DP
encoders.  Take advantage of that to prevent cases
where we may end up sharing a PPLL between DP and
non-DP which won't work.  Also clean up the code
a bit.

v2: - fix missing pll_id assignment in crtc init
v3: - fix DP PPLL check
    - document functions
    - break in main encoder search loop after matching.
      no need to keep checking additional encoders.

fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=54471

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
[bwh: Backported to 3.2: drop the DCE6 case]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>