drm/i915: Use internal class when counting engine resets

Commit 50357944 ("drm/i915/gsc: Mark internal GSC engine with reserved uabi class")
made the GSC0 engine not have a valid uabi class and so broke the engine
reset counting, which in turn was made class based in cb823ed9

 ("drm/i915/gt: Use intel_gt as the primary object for handling resets").

Despite the title and commit text of the latter is not mentioning it (and
has left the storage array incorrectly sized), tracking by class, despite
it adding aliasing in hypthotetical multi-tile systems, is handy for
virtual engines which for instance do not have a valid engine->id.

Therefore we keep that but just change it to use the internal class which
is always valid. We also add a helper to increment the count, which
aligns with the existing getter.

What was broken without this fix were out of bounds reads every time a
reset would happen on the GSC0 engine, or during selftests when storing
and cross-checking the counts in igt_live_test_begin and
igt_live_test_end.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: dfed6b58

 ("drm/i915/gsc: Mark internal GSC engine with reserved uabi class")
[tursulin: fixed Fixes tag]
Reported-by: Alan Previn Teres Alexis <alan.previn.teres.alexis@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231201122109.729006-2-tvrtko.ursulin@linux.intel.com