drm/i915/perf: Move OA regs to their own header

The OA unit registers are only used by the perf code; move them to their
own header file.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220127234334.4016964-2-matthew.d.roper@intel.com