x11r7: move X.org server style option

The selection between "modular" server and "Kdrive" server really
belongs as a sub-option of the X.org server itself, rather than as a
global x11r7 option. So we move it under the X.org server option.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>