drm/radeon: rework pll selection (v3)

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>
Cc: stable@vger.kernel.org