package/mesa3d: only enable full OpenGL when a DRI driver is enabled

Full OpenGL is only provided when:
  - a DRI driver is enabled,
  - and X.Org is enabled

Since DRI driver depend on X.Org being selected, this means that we
can (have to) enable full OpenGL as soon as a DRI driver is enabled.

On the other hand, Gallium drivers, which currently do enable full
OpenGL, only really provide OpenGL EGL and GLES, not full OpenGL.

So we exclude Gallium drivers when checking whether to enable full
OpenGL in mesa3d.

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