drm: fix returning -EINVAL on setmaster if another master is active

drm: fix returning -EINVAL on setmaster if another master is active

We link every DRM "file_priv" to a "drm_master" structure. Currently, the
drmSetMaster() call returns 0 when there is _any_ active master associated
with the "drm_master" structure of the calling "file_priv". This means,
that after drmSetMaster() we are not guaranteed to be DRM-Master and might
not be able to perform mode-setting.

A way to reproduce this is by starting weston with the DRM backend from
within an X-console (eg., xterm). Because the xserver's "drm_master" is
currently active, weston is assigned to the same master but is inactive
because its VT is inactive and the xserver is still active. But when
"fake-activating" weston, it calls drmSetMaster(). With current behavior
this returns "0/success" and weston thinks that it is DRM-Master, even
though it is not (as the xserver is still DRM-Master).
Expected behavior would be drmSetMaster() to return -EINVAL, because the
xserver is still DRM-Master. This patch changes exactly that.

The only way this bogus behavior would be useful is for clients to check
whether their associated "drm_master" is currently the active DRM-Master.
But this logic fails if no DRM-Master is currently active at all. Because
then the client itself would become DRM-Master (if it is root) and this
makes this whole thing useles.

Also note that the second "if-condition":
  file_priv->minor->master != file_priv->master
is always true and can be skipped.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>