mesa3d: enable DRI drivers for xorg-less configurations

mesa3d can in fact build the DRI infrastructure/drivers just fine
without lingering x11 libs around, it just needs libdrm & friends which
are already accounted for.

So make the libGL (full OpenGL) providing abilities dependant on x11
being present.

It serves it's purpose for EGL+GLES hardware acceleration, and can in
fact be built without them, but it's probably not very useful (still,
leave the option for people inclined to do so).

Simple test defconfig:

BR2_arm=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_PACKAGE_MESA3D=y
BR2_PACKAGE_MESA3D_DRI_DRIVER_SWRAST=y
BR2_PACKAGE_MESA3D_DRI_DRIVER_NOUVEAU=y
BR2_PACKAGE_MESA3D_DRI_DRIVER_RADEON=y
BR2_PACKAGE_MESA3D_OPENGL_EGL=y
BR2_PACKAGE_MESA3D_OPENGL_ES=y

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