drm: correctly update connector DPMS status in drm_fb_helper

We don't currently update the DPMS status of the connector (both in the
connector itself and the connector's DPMS property) in the fb helper
code.  This means that if the kernel FB core has blanked the screen,
sysfs will still show a DPMS status of "on".  It also means that when X
starts, it will try to light up the connectors, but the drm_crtc_helper
code will ignore the DPMS change since according to the connector, the
DPMS status is already on.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=28436 (the annoying
"my screen was blanked when I started X and now it won't light up" bug).

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>