drm: Check for lessee in DROP_MASTER ioctl

Don't let a lessee control what the current DRM master is set to;
that's the job of the "real" master. Otherwise, the lessee would
disable all access to master operations for the owner and all lessees
under it.

This matches the same check made in the SET_MASTER ioctl.

Change-Id: I0c7208bceecd456ee1deb249fc86a609d08169ea
Signed-off-by: Keith Packard <keithp@keithp.com>
Fixes: 2ed077e467ee ("drm: Add drm_object lease infrastructure [v5]")
Cc: <stable@vger.kernel.org> # v4.15+
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180119015159.1606-1-keithp@keithp.com
Git-commit: 761e05a702f5d537ffcca1ba933f9f0a968aa022
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>