drm/i915: Drop a spurious intel_pre_plane_update() call

Kill the extra intel_pre_plane_update() I accidentally added in

 commit 852eb00d
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Wed Jun 24 22:00:07 2015 +0300

    drm/i915: Try to make sure cxsr is disabled around plane
    enable/disable

This fixes a load of warnings from the frontbuffer tracking.

Testcase: igt/kms_frontbuffer_tracking/fbc-1p-rte
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>