drm: Move drm_update_vblank_count()

Move drm_update_vblank_count() to avoid forward a declaration.
No functional change.

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>