drm/i915/vlv: use PIPE_START_VBLANK interrupts on VLV

This fixes a mismatch between our vblank enable code and our IRQ
handler.  Also, since vblank start events come in before page flips
reliably, it also fixes the kms_flip plain-flip test on my BYT system.

Spotted-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>