drm/i915: Parameterize R_PWR_CLK_STATE register definition

At the moment we only use R_PWR_CLK_STATE in the context of the RCS
engine, but upcoming support for compute engines will start using
instances relative to the CCS engine base offsets.  Let's parameterize
the register and move it to the engine reg header.

Cc: Jani Nikula <jani.nikula@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/20220127234334.4016964-4-matthew.d.roper@intel.com