Keep waiting for a pending flip if drm_handle_event returns 0

drm_wait_pending_flip stopped waiting if drm_handle_event returned 0,
but that might have processed only some unrelated DRM events. As long as
the flip is pending, we have to keep waiting for its completion event.

Noticed while working on the previous fix.

(Ported from amdgpu commit 9045fb310f88780e250e60b80431ca153330e61b)