drm/i915: Sanity check PPS HW state

The wait for panel status helper will only function correctly if the
HW panel timings are programmed correctly. Returning prematurely from
this helper may lead to obscure bugs later, so sanity check the HW
timing registers.

v2:
- Check the T8, T9 fields too, we do program them (Ville)

CC: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466096506-11937-1-git-send-email-imre.deak@intel.com