backends/drm: Notify about failed frames if there are actually pending frames

In case a modeset needs to be performed, the drm backend will test all
pipelines to ensure that new mode won't cause any bandwidth issues on
other outputs, etc.

To do that, it may delay presenting frames. If the new configuration
doesn't work, it needs to notify about failed frames.

However, the relevant code that notifies the RenderLoop about failed
atomic commits doesn't check if there's actually a pending modeset
present.

When switching between VTs, systemd can revoke master permissions from
kwin. To make things even more trickier, kwin can try to present a frame
in that short time span.