drm/i915: Do not touch the PCH SSC reference if a PLL is using it

Our PCH refclk init code currently assumes that the PCH SSC reference
can only be used for FDI. That is not true and it can be used by
SPLL/WRPLL for eDP SSC or clock bending as well. Before we go
reconfiguring it let's make sure no PLL is currently using the PCH
SSC reference.

For some reason the hw is not particularly upset about losing
the clock if we immediately follow up with a modeset. Can't
really explain why nothing times out during the crtc disable
at least, but that's what the logs say. With fastboot the
story is quite different and we lose the entire display if
we turn off the PCH SSC reference when it's still being used.

Since we totally skip configuring the PCH SSC reference it
may not be in the proper state for FDI. Hopefully that won't
be a problem in practice.

We really should move this code to be part of the modeset seqeuence
and properly deal with the potentially conflicting requirements
imposed on PLL reference clocks. But that requires actual work.
Let's toss in a TODO for that.

v2: Pimp the commit message with the fastboot vs. not
    details

Cc: Julius B. <freedesktop@blln.gr>
Cc: Johannes Krampf <johannes.krampf@gmail.com>
Tested-by: Johannes Krampf <johannes.krampf@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108773
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190604200933.29417-1-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

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