backends/drm: store the crts a legacy commit happens on in the commit object

The crtc in the drm pipeline may change between a pageflip being requested and the pageflip event happening. If it does change, it could change to a nullptr, in which case KWin would then crash.

This should fix https://crash-reports.kde.org/organizations/kde/issues/4603

(cherry picked from commit 34b830f8)

Co-authored-by: Xaver Hugl xaver.hugl@gmail.com