imx6: set preferred version for gpu drivers

Including older gpu drivers in b2qt layer, caused it to be used
in all machined. Set preferred version to latest for all but
apalis that needs to older one.

The matching kernel version is set on upstream layer, so no need
to specify it here.

Change-Id: Ia4ebfbc3b66d481f9e2b5a996d242f853b03a464
Reviewed-by: Andy Nichols <andy.nichols@digia.com>