package/mesa3d: always enable openGL

Currently, we only pass --enable-opengl is at least one DRI driver is
enabled, since full OpenGL is only possible with a DRI driver (and the
X.Org stack). Otherwise, we pass --disable-opengl on the assumption that
OpenGL is not possible.

But mesa3d's ./configure is a bit weird: enabling OpenGL-ES and diabling
full OpenGL is an error (see autobuild failure below).

It turns out that, if there is no DRI driver enabled, but --enable-opengl
is passed, ./configure will behave properly wrt OpenGL-ES, and will not
build the full OpenGL libgl.

Thanks to Paul for explaining this.

Fixes:
    http://autobuild.buildroot.net/results/61c/61c1c566dc0e829cb663ca30b1fd6cf9cc6cd931/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>