backends/drm: Clear damage journal when switching between compositing and direct scanout

When returning back from direct scanout to compositing, the whole frame
has to be repainted because the most recent buffer in the EGL swapchain
contains outdated contents now.

This change works around it by purging the DamageJournal when scanning
out a buffer. However, a nicer solution would be to assign a unique
sequence number to every frame and use that to index the entries in the
DamageJournal. It's a nicer design. With such absolute frame numbers,
the DamageJournal could properly detect discontinueties.

Given the final release is in close proximity, this change doesn't go
after such a refactoring.