drm: Extract drm_master_open

And pull out the primary_client check to make it really obvious that
this can't happen on control/render nodes. Bonus that we can avoid the
master lock in this case.

v2: Don't leak locks on error path (and simplify control flow while
at it), reported by Julia.

Cc: Julia Lawall <julia.lawall@lip6.fr>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465930269-7883-6-git-send-email-daniel.vetter@ffwll.ch