drm/i915: Parameterize ECOSKPD

Combine the separate render and blitter register definitions into a
single definition.  We already know we have some workarounds on an
upcoming platform that will need to update the ECOSKPD register for
other engines too, so this helps pave the way for that.

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/20220111051600.3429104-4-matthew.d.roper@intel.com