drm/i915: Count engine instances per uabi class

This will be useful to have at hand in a following patch.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220401142205.3123159-8-tvrtko.ursulin@linux.intel.com