Wait for pending flips in drmmode_output_set_tear_free

This prevents a nested call to drmHandleEvent, which would hang.

Fixes hangs when disabling TearFree on a CRTC while a DRI3 client is
page flipping.

(Ported from amdgpu commit 04947b83cce3a7782e59dece2c7797cc396c1e05)
Acked-by: Alex Deucher <alexander.deucher@amd.com>