sema: GR ring autopin must not trust an early base or a pre-existing pin

Two failure modes kept every GR channel dark despite the ring layout being
known:

- The CE region size was latched on the first autopin attempt, which can run
  before all CE channels are registered; a too-small base then points every
  GR candidate at a chunk whose entry 0 is zeros, and it stays wrong for the
  life of the boot. Recompute the base from the live CE ranks on every pass.
- An existing pin for a ring VA (e.g. from a convention guess) made the
  autopin mark the rank done without validating the ring - a wrong USERD is
  then never corrected. Validate the candidate chunk (entry 0 decode + USERD
  put/get) first and only fall back to trusting an existing pin when the
  chunk does not validate; on success overwrite the pin.

Also log one diagnostic line per GR rank when the chunk does not validate
yet, so a silent autopin failure is visible in dmesg.

Live: fresh boot, no manual ring pins - 20/20 GR channels auto-pinned
(correct USERD 0x142000 + 0x3000*i) and ran entry 0 (get=1 sub=1) once the
push pages were bridged.

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