drm/udl: Make sure to get correct endian keys from vendor descriptor

This is a port of
commit b49f184b640dcfab7ede394cf2a1ff4fe3d154f5
Author: Ben Collins <bcollins@ubuntu.com>
from udlfb to udl kms driver.

The driver was not using le16_to_cpu when reading keys from the vendor
descriptor, causing incorrect parsing. Mainly, sku_pixel_limit was not
being parsed on big-endian systems. This would result in a blank screen
on big-endian CPUs where the DL chips's max mode was smaller than the
monitor's native mode.

Signed-off-by: Dave Airlie <airlied@redhat.com>