drm/i915: split out intel_display_reg_defs.h

Split out the display register helper macros to a separate file. For
now, include it from i915_reg.h, but note that there are already files
that don't need i915_reg.h, such as intel_audio.c.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/3af47193ff5219b6d2cfe353b752ec4bb44de4f1.1668008071.git.jani.nikula@intel.com