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A documentation error and bug in BCMVideo combined with an incompatible change
in the VCHIQ interface, resulting in blank video output.

* In GenCmd_GetConfigInt, in the non-error case, Z is clear (not set) if the
  value read is nonzero. Many of the callers (correctly) only store the result
  if NE was returned, i.e. Z clear.
* In the parse error case, where it constructs the return flags using an
  MSR, it wasn't setting Z, so the callers were storing the corrupted r0
  (actually a pointer to the query string) into the workspace in such cases.
* The firmware has changed how it reports unknown values. It now responds
  "<name> is unknown" rather than "<name>=0", and this meant it was being
  considered as a parsing error, hence triggering the dormant bug above.
* GenCmd_GetConfigInt has now gained special-case handling of " is unknown"
  results, and treats them the same as "=0".

Tested on a Pi 4 with the latest firmware.