sna: Discard the per-crtc TearFree back buffer on resize

When we mix TearFree and per-crtc pixmaps (e.g. for RandR transformations),
we stash the old buffer on the CRTC for double buffering. However, this
buffer needs to be reallocated when we change output resolutions, as the
CRTC size may change.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>