drm/i915: Clear display interrupt before enabling when turning on the power well

drm/i915: Clear display interrupt before enabling when turning on the power well

For a bit of extra paranoia make sure the display irqs are all cleared
before we enabled them when turning on the power well. This should
really be the case already since the power well was off which resets
everything.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460382992-28728-6-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>