drm/i915: Restore vblank interrupts earlier

Plane sanitation needs vblank interrupts (on account of CxSR disable).
So let's restore vblank interrupts earlier.

v2: Make it actually build
v3: Add comment to explain why we need this (Daniel)

Cc: stable@vger.kernel.org
Cc: Dennis <dennis.nezic@utoronto.ca>
Tested-by: Dennis <dennis.nezic@utoronto.ca>
Tested-by: Peter Nowee <peter.nowee@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105637
Fixes: b1e01595

 ("drm/i915: Redo plane sanitation during readout")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181003144951.4397-1-ville.syrjala@linux.intel.com