drm: Fix confusing debug message in drm_update_vblank_count()

Now that drm_update_vblank_count() can be called even when we're not
about to enable the vblank interrupts we shouldn't print debug messages
stating otherwise.

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