drm/i915: Parameterize PWRCTX_MAXCNT

Rather than having separate definitions for each engine, create a single
parameterized macro that takes the engine base offset.  This will also
ensure we get to the proper offset if we ever need to use these
registers on newer platforms (where the media engine offsets have
changed).

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220111051600.3429104-3-matthew.d.roper@intel.com