drm/i915: Try to suppress more spurious PCH underruns on ILK-IVB

drm/i915: Try to suppress more spurious PCH underruns on ILK-IVB

My ILK seems to generate a spurious PCH underrun with most interlaced
HDMI modes. Add a second vblank wait to avoid it.

We have seen some spurious PCH underruns still in CI as well, some
of which seem to be progressive DP. The logs also point towards some
spurious underrins with progressive HDMI on SNB. While I don't have
a solid explanation for those let's try to kill all the birds with one
stone and always do the double wait.

Buzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106387
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180524190406.2973-1-ville.syrjala@linux.intel.com
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>