drm/i915/gt: Move engine registers to their own header

Let's continue breaking up and cleaning up the massive i915_reg.h file
by moving all registers that are defined in relation to an engine base
to their own header.

There are probably a bunch of other "engine registers" that we haven't
moved yet (especially those that belong to the render engine in the
0x2??? range), but this is a relatively straightforward first step.

Cc: Jani Nikula <jani.nikula@linux.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/20220111051600.3429104-8-matthew.d.roper@intel.com