ecore_drm2: Don't try to disable all devices when session deactivates

ecore_drm2: Don't try to disable all devices when session deactivates

We get this callback after we've lost the drm device to logind, so
deactivating stuff here will just generate a lot of ERR messages
and break our internal book-keeping.

Instead, we just turn on DPMS on session activation instead of trying
to go through the output enable path (that will bail if it's already
enabled)

This could potentially result in a display that's enabled and DPMS
off being switched back on during session activation - if that's a real
problem we can restore the previous dpms state instead...

@fix T5483