drm/i915/display: add typedef for intel_reg_t and use it

drm/i915/display: add typedef for intel_reg_t and use it

Add a typedef alias intel_reg_t for i915_reg_t, and use it exclusively
in display code. The goal is to eventually define a distinct type for
display, but for now just use an alias.

In a handful of places include intel_display_reg_defs.h instead of
i915_reg_defs.h to get the definition, and isolate the i915_reg_defs.h
include there.

Reviewed-by: Michał Grzelak <michal.grzelak@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/f5bb0122143c39ce7f9ce8a1605a7637e569f7f8.1777972548.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>