drm/i915: Fix assert_plane warning during FDI link train

assert_plane_enabled() is now triggering during FDI link train because
we no longer enable planes that early.

This problem got introduced in:
 commit a5c4d7bc
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Fri Mar 7 18:32:13 2014 +0200

    drm/i915: Disable/enable planes as the first/last thing during modeset on ILK+

Just drop the assert since we shouldn't need planes for link training.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Squash in fixup for now unused plane local variable, reported
by 0-day tester.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>