drm/i915: delay master/sarea deref for legacy ioctls

... past the check for DRIVER_MODESET. Avoids races with userspace
opening a master and our sarea setup.

Cc: Signed-off-by: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>