drm/i915: Fold IRQ pipe masks

Fold IRQ pipe masks into one loop instead of hardcoding per pipe.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1507630626-23806-4-git-send-email-mika.kahola@intel.com