drm/i915: replace ad-hoc dual-link lvds checks

... with is_dual_link_lvds introduced in

commit b0354385
Author: Takashi Iwai <tiwai@suse.de>
Date:   Tue Mar 20 13:07:05 2012 +0100

    drm/i915: Check VBIOS value for determining LVDS dual channel mode, too

All these checks predate this commit and have simply been overlooked.
Since we don't support switching between single-link and dual-link
modes anyway, this different checks could at best only get in the way
of refactorings, and in the worst case cause inconsistencies.

v2: Update the comment, we now have a solid way to figure out whether
we need dual-link lvds or not (falling back to vbt values as a last
resort). We still don't know how to switch between dual-link and
single link so leave that part intact. I'm not sure though whether
switching between these two modes makes any sense - we always drive
the panel at its fixed mode (with a fixed bpc) anyway ...

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>