drm/i915: Fix LVDS fixed-mode regression from 219adae1

Commit 219adae1

 cached the EDID found during LVDS init, but in the
process prevented the init routine from discovering the preferred
fixed-mode for the panel. This was causing us to guess the correct mode,
which sometimes is wide of the mark.

Reported-and-tested-by: Jon Masters <jonathan@jonmasters.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>