drm/i915: Skip vblank waits for cursor updates when watermarks dont need updating

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