platforms/drm: delay presentation for modesets

Currently KWin is combining modesets with presentation, which causes problems
when multiple monitors are used and crtcs need to be switched around, because
taking away a CRTC from another output causes the driver to disable the
other output. In order to avoid such problems, delay presentation until
all pipelines are ready to present and then do a modeset with a single atomic
commit. To process the resulting page flip events properly this commit also
ports KWin to page_flip_handler2 and changes how the pageFlipped and
notifyFrameFailed signals are processed.