drm/i915: Kill intel_flush_primary_plane()

The primary plane frobbing was removed from the sprite code in
 commit ecce87ea
 Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
 Date:   Tue Apr 21 17:12:50 2015 +0300

    drm/i915: Remove implicitly disabling primary plane for now

but the intel_flush_primary_plane() calls were left behind. Replace them
with straight forward POSTING_READ() of the sprite surface address
register.

The other user of intel_flush_primary_plane() is g4x_disable_trickle_feed()
where we can just inline the steps directly.

This allows intel_flush_primary_plane() to be killed off.

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