drm/i915/uncore: Do GT FIFO checks in early sanitize and forcewake get

We're mixing up the GT FIFO debug checks (overflows and such)
with RMbus unclaimed register checks. The two are quite different
things as RMbus is only relevant for display registers, and the
GT FIFO only relevant for GT registers.

Split the GT FIFO debugs out from the unclaimed register logic
and just do the checks during forcewake_get() and early init.
That is still sufficient to detect if any errors have happened.

Any errors would anyway be caused by overflowing the FIFO
rather than accessing specific registers, so trying to figure
out exactly when the error happened isn't particularly useful.
To fix such issues we'd rather have to do something to slow down
the rate at which registers are accessed (eg. increase
GT_FIFO_NUM_RESERVED_ENTRIES or something).

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260323101609.8391-3-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>