backends/drm: attempt a modeset on output disabling

When modesets are necessary, they are attempted when an output on the given
GPU gets presented. With multi-gpu setups however, the situation can arise
where there is only one disabled output on a GPU; in that case KWin eternally
waits and never properly turns off the display.
In order to work around this, explicitly call DrmGpu::maybeModeset when
an output gets disabled.

BUG: 449878
FIXED-IN: 5.24.4


(cherry picked from commit 658457df)