dri: Simplify use of driConcatConfigs

dri: Simplify use of driConcatConfigs

If either argument to driConcatConfigs(a, b) is null or the empty list,
then simply return the other argument as the resultant list.

All callers were accomplishing that same behavior anyway. And each caller
accopmplished it with the same pattern. So this patch moves that external
pattern into the function.

Reviewed-by: <eric@anholt.net>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>