drm/{i915, xe}: Extract pcode definitions to common header

drm/{i915, xe}: Extract pcode definitions to common header

There are certain register definitions which are commonly shared
by i915, xe and display. Extract the same to a common header to
avoid duplication.

Move GEN6_PCODE_MAILBOX to common pcode header to make intel_cdclk.c
free from including i915_reg.h.

v3: Include pcode header as required, instead in i915_reg.h (Jani)

v2: Make the header granular and per feature (Jani)

Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patch.msgid.link/20260205094341.1882816-6-uma.shankar@intel.com