drm/i915: Add readout and state check for pch_pfit.force_thru

drm/i915: Add readout and state check for pch_pfit.force_thru

Convert the HSW pch_pfit.force_thru to a proper state variable
with readout and accompanying pipe conf check. Makes the logic
a bit more straightforward, and hopefully prevents some
breakage in the future.

'force_thru' is probably not the best name for this, but I
didn't manage to come up with anything better either, so I
left it alone.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190425162906.5242-2-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>