compositor-drm: Don't page flip before a mode is set

compositor-drm: Don't page flip before a mode is set

The function drm_output_start_repaint_loop() unconditionally issues a
page flip, even if the crtc for that output has not been enabled yet.
That causes the page flip to fail, and drm_output_repaint() is never
called.

Solve this by bypassing the initial page flip if the output needs a
mode set.

This has the caveat of affecting latency predictability for that first
frame and when a "driver" mode fullscreen surface causes a mode set.
However, on both cases the mode set would take an unpredictable amount
of time anyway.

https://bugs.freedesktop.org/show_bug.cgi?id=63812
https://bugs.freedesktop.org/show_bug.cgi?id=64183