gbm: track buffer format through DRI drivers

gbm: track buffer format through DRI drivers

GBM needs the buffer format in order to communicate with DRM and clients
for things like scanout.

So track the DRI format requested in the various back ends and use it to
return the DRI format back to GBM when requested.  GBM will then map
this into the GBM surface type (which is in turn based on the DRM fb
format list).

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>