Switch reference counting to GEM instead of TTM

["Christian König" <ckoenig.leichtzumerken@xxxxxxxxx>]

Hello everyone,

to make use of drm_exec it is necessary to have TTM drivers actually use
the GEM reference count instead of the TTM one.

This patch set is a start to do that. It switches all uses of
ttm_bo_get/put to the GEM counterpart and then makes ttm_bo_get private
to TTM.

Netx step is to completely remove the TTM refcounting.

Please review and/or comment.

Cheers,
Christian.