package/x11r7/xserver_xorg-server: Fix libdrm dependency handling

Fixes
http://autobuild.buildroot.net/results/2e1/2e12f1a91812f027d89fcceeb41d6c88ef7b9d32/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>