Store FB for each CRTC in drmmode_flipdata_rec

We were only storing the FB provided by the client, but on CRTCs with
TearFree enabled, we use a separate FB. This could cause
drmmode_flip_handler to fail to clear drmmode_crtc->flip_pending, which
could result in a hang when waiting for the pending flip to complete. We
were trying to avoid that by always clearing drmmode_crtc->flip_pending
when TearFree is enabled, but that wasn't reliable, because
drmmode_crtc->tear_free can already be FALSE at this point when
disabling TearFree.

Now that we're keeping track of each CRTC's flip FB separately,
drmmode_flip_handler can reliably clear flip_pending, and we no longer
need the TearFree hack.

(Ported from amdgpu commit 9b6782c821e0bdc53336d98f87ddde752faf7902)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>