drm/i915: Simplify ilk-ivb underrun suppression

drm/i915: Simplify ilk-ivb underrun suppression

Let's suppress the underruns around every modeset sequence instead
of trying to avoid it. Planes are disabled at this point anyway so
we don't really gain anything from keeping the underrun reporting
enabled. Also for PCH ports we already suppress all underruns here
anyway so trying avoid it for the CPU eDP doesn't seem all that
important.

Maybe this gets rid of some lingering spurious underruns?

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180524190406.2973-2-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>