Revert "drm: drop redundant drm_file->is_master"
This reverts commit 48ba8137. Thanks to Chris: "drm_file->is_master is not synomous with having drm_file->master == drm_file->minor->master. This is because drm_file->master is the same for all drm_files of the same generation and so when there is a master, every drm_file believes itself to be the master. Confusion ensues and things go pear shaped when one file is closed and there is no master anymore." Conflicts: drivers/gpu/drm/drm_drv.c drivers/gpu/drm/drm_stub.c