drm: Only do the hw.lock cleanup in master_relase for !MODESET

Another place gone where modern drivers could have hit
dev->struct_mutex.

To avoid too deeply nesting control flow rework it a bit.

v2: Review from Chris:
- remove spurious newline.
- fix file_priv->master like for the !file_priv->is_master case.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
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@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466148814-8194-2-git-send-email-daniel.vetter@ffwll.ch