In legacy cursor updates we need the extra vblank waits if we update
watermarks, and then we cannot skip the vblank for cursors.

This is why for < gen9 we disabled the cursor fastpath, but we can skip
the wait when post vblank watermarks are untouched.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170919121419.13708-2-maarten.lankhorst@linux.intel.com