mesa3d: Give possibility to external backends to enable DRI/Gallium

mesa3d: Give possibility to external backends to enable DRI/Gallium

Thus, it is possible to force compilation of Gallium/DRI support in
mesa3d even if no Gallium/DRI drivers are selected. This is intended to
be used by external OpenGL provider (especialy further imx6 support).

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Reviewed-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Tested-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>