Abort scanout_update_pending event when possible

We don't need to wait for a non-TearFree scanout update before scanning
out from the screen pixmap or before flipping, as the scanout update
won't be visible anyway. Instead, just abort it.

(Ported from amdgpu commit 36d01989cd842588f12fdae5b2cba5fdcf9c91dd)
Acked-by: Alex Deucher <alexander.deucher@amd.com>