mesa: Fix a case when gbm is enabled but DRIDRIVERS is not defined
This is a typical scenario with arm platforms, with meson if DRIDRIVERS
is empty then dri is disabled automatically, but gbm needs dri2 enabled
for it to have proper backend, in autotools we could use --enable-gbm
--enable-dri and get correct behaviour but with meson enabling dri2 only
works if DRIDRIVERS is not empty
This fixes mesa-gl build for rpi machines when using userland graphics
Signed-off-by: Khem Raj <raj.khem@gmail.com> Signed-off-by: Andrei Gherzan <andrei@gherzan.ro> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
