drm: Clear up master tracking booleans

- is_master can be removed, we can compute this by checking allowed_master
  (which really just tracks whether a master struct has been allocated
  for this fpriv in either open or set_master), and whether the fpriv is
  the current master on the device.

- that frees up is_master as a good replacement name for allowed_master.
  With that it's clear that it tracks whether the fpriv is a master (with
  possibly clients attached to it and authenticated against it), and that
  one of those fprivs with is_master set is the current master.

v2: Fix kerneldoc for is_master (Emil).

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466499262-18717-10-git-send-email-daniel.vetter@ffwll.ch