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

Currently i915 driver checks [PCH_]LVDS register bits to decide
whether to set up the dual-link or the single-link mode.  This relies
implicitly on that BIOS initializes the register properly at boot.
However, BIOS doesn't initialize it always.  When the machine is
booted with the closed lid, BIOS skips the LVDS reg initialization.
This ends up in blank output on a machine with a dual-link LVDS when
you open the lid after the boot.

This patch adds a workaround for that problem by checking the initial
LVDS register value in VBT.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=37742


Tested-By: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>