drm/i915/dsi: Assert that vfp+vsync+vbp == vtotal on BXT/GLK

drm/i915/dsi: Assert that vfp+vsync+vbp == vtotal on BXT/GLK

Windows code seems to do some dodgy looking +/-1 adjustments
to some of the vertical timings. Not sure if that's correct
or not, but let's at least cross check some of the values
we got from the hardware to double check whether the GOP
did the same or not.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250314150136.22564-3-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>