drm/radeon/kms: simplify hotplug handler logic

In the hotplug handler, just use the drm dpms functions.
If the monitor is plugged in, turn it on, if it's not,
turn it off.  This also reduces power usage by turning
off the encoder and crtc when the monitor is unplugged.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>