drm/i915: Check for underruns after crtc disable

drm/i915: Check for underruns after crtc disable

To get a better idea if underruns occurred during crtc disabling,
let's check for them explicitly. This helps in cases where the
error interrupt isn't active, or there is no underrun interrupt
support at all.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448050160-14124-3-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>