Add generic __to_intel_display() macro that accepts either struct
drm_i915_private * or struct intel_display *. This is to be used for
transitional stuff that eventually needs to be converted to use struct
intel_display *, and therefore is not part of to_intel_display().

Add new intel_display_conversion.h to host the helper to avoid
duplication between xe and i915 drivers.

v2: put it in the new header (Rodrigo)
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a999ff8183659a4df68d439ebd31c19b5c56852a.1713358679.git.jani.nikula@intel.com