x11r7: remove useless depends on of X.org drivers

All X.org server drivers are already enclosed in a if
BR2_PACKAGE_XSERVER_xorg .. endif block. Now that this option is only
set if a X.org server is enabled, there is no need for each individual
driver to depend on BR2_PACKAGE_XSERVER_XORG.

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>