Remove DPMS-on when going offscreen

Remove DPMS-on when going offscreen

Forcing DPMS on when we lose our session may force an expensive modeset
operation, which is pointless if the next consumer (another compositor,
or the console) is going to do a modeset. These should force DPMS on
regardless.

This actively causes problems for the DRM backend, in that it may
actually require a repaint to set coherent state for DPMS off -> DPMS on
transitions, which is very much not what we want when going offscreen.

As DRM is the only backend which actually implements DPMS, just remove
this call.

Differential Revision: https://phabricator.freedesktop.org/D1483

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>