drm/i915/perf: Group engines into respective OA groups

Now that we may have multiple OA units in a single GT as well as on
separate GTs, create an engine group that maps to a single OA unit.

v2: (Jani)
- Drop warning on ENOMEM
- Reorder patch in the series

v3: (Ashutosh)
- Remove unused members from perf structs
- Update comments
- Update engine_supports_oa check
- Just return 1 in num_perf_groups_per_gt for now
- Set engine->oa_group to NULL to begin with

v4: Use engine_supports_oa() check in oa_init_reg_state (Ashutosh)
v5: Rebase after dropping engine_supports_oa helper

Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230323225901.3743681-5-umesh.nerlige.ramappa@intel.com