meson: build gbm

meson: build gbm

This doesn't include egl support, just dri support.

v2: - when gbm is set to 'auto', only build if a dri driver is also
      enabled
    - Fix conditional to check for x11 modules with vulkan as well as
      with dri drivers
v3: - Set pkgconfig libraries.private value

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>