Revert "drm: drop redundant drm_file->is_master"

Revert "drm: drop redundant drm_file->is_master"

This reverts commit e82486b6f36a4629d1864e6e166c6bc72ecf8f84.

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