drm: allow changing DPMS mode

drm: allow changing DPMS mode

The drm_atomic_helper_connector_dpms
will set the connector back the old DPMS state
before returning. This makes it impossible to change
DPMS state of the connector.

Fixes: 0853695c3ba46f97dfc0b5885f7b7e640ca212dd
v2: edit of commit message
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Eric Engestrom <eric.engestrom@imgtec.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: dri-devel@lists.freedesktop.org
Cc: <drm-intel-fixes@lists.freedesktop.org>
Signed-off-by: Marta Lofstedt <marta.lofstedt@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161205120408.13056-1-marta.lofstedt@intel.com
(cherry picked from commit 8f5040e421ca4bbd8cd5216dec509da188eb74bc)