drm/i915: Remove stale comment from __intel_set_mode()

Since the following commit, the PLL calculations are done earlier, so
the code following the comment doesn't do anything PLL or encoder
related. It only updates the primary plane now.

commit f3019a4d
Author: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Date:   Wed Oct 29 11:32:37 2014 +0200

    drm/i915: Remove crtc_mode_set() hook

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>