drm/radeon/kms: use LCD physical size from vbios tables if available

Some systems have the LCD width and height in mm available in the
LCD info table.  Use this info if there is no EDID to provide it.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>