commit fd3a4024 ("drm/i915: Rip out legacy page_flip completion/irq
handling") removed the code to hande the flip done/pending interrupts,
but it failed to actually disable/mask those interrupts. Let's do that
now.

Also remove a stale comment that was left behind.

Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170818183705.27850-2-ville.syrjala@linux.intel.comReviewed-by: Imre Deak <imre.deak@intel.com>