drm/i915: Introduce i915_reg_defs.h

We'd like to start splitting i915_reg.h into various domain-specific
register files and cleaning them up.  Let's move the basic macros and
type definitions to their own header file that can be including in each
of the new split headers.

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/20220111051600.3429104-7-matthew.d.roper@intel.com