drm/i915: Return the frontbuffer flip to enable intel_crtc_enable_planes.

Without this frontbuffer flip when enabling planes PSR got compromised
and wasn't being enabled waiting forever on the flush that never
arrived.

Another solution would to create a enable_cursor function and split this
frontbuffer flip among the different plane enable and disable functions.
But if necessary this can be done in a follow up work. For now let's
just fix the regression.

It was removed by:

commit 87d4300a
Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Date:   Tue Apr 21 17:12:54 2015 +0300

    drm/i915: Move intel_(pre_disable/post_enable)_primary to intel_display.c, and use it there.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>