drm/radeon/kms: fix dpms state on resume

When suspending, we turn the display hw off, at resume the screen will stay black.
This patch turn it on. Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=16180



Signed-off-by: Cedric Godin <cedric.godin@skynet.be>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>