Only update drmmode_crtc->flip_pending after actually submitting a flip

And only clear it if it matches the framebuffer of the completed flip
being processed.

Fixes

 (WW) RADEON(0): flip queue failed: Device or resource busy
 (WW) RADEON(0): Page flip failed: Device or resource busy
 (EE) RADEON(0): present flip failed

due to clobbering drmmode_crtc->flip_pending.

Reproducer: Enable TearFree, run warzone2100 fullscreen, toggle
Vertical sync on/off under Video Options. Discovered while investigating
https://bugs.freedesktop.org/109364 .

(Ported from amdgpu commit e72a02ba1d35743fefd939458b9d8cddce86e7f5)