drm: Kick start vblank interrupts at drm_vblank_on()

If the user is interested in getting accurate vblank sequence
numbers all the time they may disable the vblank disable timer
entirely. In that case it seems appropriate to kick start the
vblank interrupts already from drm_vblank_on().

v2: Adapt to the drm_vblank_offdelay ==0 vs <0 changes

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>