drm/i915: Honour LVDS sync polarity from EDID

The i915 driver normally assumes the video bios has configured several
of the LVDS panel registers, and it just inherits the values. If the
vbios has not run, several of these will need to be setup. So we need to
check that the LVDS sync polarity is correctly configured per any
available modelines (e.g. EDID) and adjust if not, issuing a warning as
we do.

Signed-off-by: Mark Hayter <mdhayter@chromium.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>