drm/i915/perf: Configure OAR for specific context

Gen12 supports saving/restoring render counters per context. Apply OAR
configuration only for the context that is passed in to perf.

v2:
- Fix OACTXCONTROL value to only stop/resume counters.
- Remove gen12_update_reg_state_unlocked as power state is already
  applied by the caller.

v3: (Lionel)
- Move register initialization into the array
- Assume a valid oa_config in enable_metric_set

Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Fixes: 00a7f0d7 ("drm/i915/tgl: Add perf support on TGL")
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191206194339.31356-2-umesh.nerlige.ramappa@intel.com