directfb: fixup input/gfx drivers handling after 0e4d25ff

The input/gfx drivers handling got broken by the ':=' -> '=' conversion,
as we redefine the variables in terms of themselves, causing recursion.

Fix it by directly assigning the result to DIRECTFB_CONF_OPT instead.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>