drm: Pass flags to drm_update_vblank_count()

We'll soon have use for the 'flags' in drm_update_vblank_count() so pass
it in.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>