package/x11r7/xserver_xorg-server: Relax dependency on libdrm

xserver_xorg-server needs libdrm only when DRI is enabled, see configure.ac,
line 1280. But since version 1.16.99.901 configure searches for optional libdrm
support in configure.ac, line 2041,

http://cgit.freedesktop.org/xorg/xserver/commit/configure.ac?id=2feda3b6b58f46cef91ea41312aac9021a703777



as well due to libdrm support being enabled by default, but the macro
PKG_CHECK_EXISTS used in line 2041 does not add libdrm-specific CFLAGS to
Makefile causing the build to fail:

In file included from dumb_bo.c:36:0:
/home/br/br7/output/host/usr/i586-buildroot-linux-uclibc/sysroot/usr/include/xf86drm.h:40:17: fatal error: drm.h: No such file or directory
 #include <drm.h>

This patch enables libdrm support only when dri support is enabled, which in
turn depends on mesa3d. Since mesa3d selects libdrm already remove that line
from Config.in as well.

Tested using this defconfig

BR2_TOOLCHAIN_BUILDROOT_LARGEFILE=y
BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
BR2_TOOLCHAIN_BUILDROOT_CXX=y
BR2_PACKAGE_XORG7=y
BR2_PACKAGE_XSERVER_XORG_SERVER=y
BR2_PACKAGE_XSERVER_XORG_SERVER_MODULAR=y

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>