drm/radeon: allow PPLL sharing on non-DP displays

If several non-DP displays use the same pixel clock
we can use the same PPLL for all of them.  If all
relevant displays have the same pixel clock, this
allows the driver to:
- use fewer PPLLs which saves power
- support more than two non-DP displays on DCE4+

The current drm modesetting infrastructure doesn't
really provide a good framework for validating combinations
that work or won't work, so it's possible you could go from
a working configuration to a non-working one by changing the
mode a one of the displays.  However, there this is better
than what was there before.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>