package/x11r7/xserver_xorg-server: add --enable-libdrm for udev

If the libdrm package is enabled, the system is built with an udev
provider and MESA3D_DRI_DRIVER is disabled this adds a --disable-libdrm
to configure options, but udev still requires libdrm and results in a
build failure since libdrm include paths aren't filled in (missing drm.h
include error).
So add --enable-libdrm for the udev scenario to counteract the previous
--disable-libdrm.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>