drm/i915: Keep vblank interrupts enabled while enabling/disabling planes

drm/i915: Keep vblank interrupts enabled while enabling/disabling planes

Because of the upcoming vblank interrupt driven watermark update
mechanism we will have use for vblank interrupts during plane
enabling/disabling. So don't call drm_vblank_off() until planes
are off, and call drm_vblank_on() just before we start to enable
the planes.

v2: Pimp commit message (Paulo)

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>