vgsp: display engine re-seeds GET on a driver ring wrap (fixes the nvidia-modeset GET-past-PUT wedge)

Root cause of the post-ScreenInit Xorg kernel-mode spin (sysrq-t: Xorg R-state
inside nvKmsIoctl -> nvkms_yield, utime frozen, stime 100%): the emulated
display engine (core channel c57d) held GET=0xcd8 PAST PUT=0xcc8 for 50 s.
gsp_disp_run only publishes GET when the parse reaches PUT exactly (by design,
per its r1630 comment), so when the guest driver wraps/resets its push
ring (PUT steps backward via nvEvoMakeRoom) the stale consumed can never
resync and NVKMS spins forever. When put < s->consumed (wrap/reset), re-seed
consumed=0/last_put=0 and republish GET=0 so the wrapped pushes are re-parsed
from the ring base; the driver's PUT is authoritative.

Validated c160 (rich args): bar0 channels now read PUT==GET (core 0x590,
window c57e 0xe2c) with no wedge and flips parse (UPDATE armed=1 -> FINISHED
posts=32..34); X advances past the spin and served xdpyinfo once. Remaining
walls (next): the push-buffer 0x51 NV_ERR_NO_MEMORY allocation lottery and
'Flip event timeout on head 0' (Xorg D in nv_drm_atomic_commit).

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