backends/drm: fix enabled+inactive check

When dpms disabled outputs get set active, they require a modeset. If after
that they are set inactive again without resetting the pipelines first,
they no longer require a modeset but still have the pending properties that
would enable a crtc - but without a framebuffer set.

To prevent this, first test the current setup as it is, and only then see
if the pipelines would work if enabled again.


(cherry picked from commit 6d9d4c19)