drm/i915: the PCH reference clocks are global, so don't clobber unconditionally

The PCH can drive several reference clocks simultaneously, and needs to
with multiple display configurations.  So we can't just clobber the
existing state everytime we set a mode, we need to take into account
what the other CRTCs are doing at the time.

Doing so fixes an issue where you'd lose the LVDS display at boot if you
had an LVDS+DP config.

[updated: init bools and check CRTC status correctly]
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>