drm/i915/display: add intel_display_conversion.c to hide stuff better

drm/i915/display: add intel_display_conversion.c to hide stuff better

The __to_intel_display() generics require the definition of struct
drm_i915_private i.e. inclusion of i915_drv.h. Add
intel_display_conversion.c with a __i915_to_display() function to do the
conversion without the intel_display_conversion.h having an implicit
dependency on i915_drv.h.

The long term goal is to remove __to_intel_display() and the
intel_display_conversion.[ch] files altoghether, and this is merely a
transitional step to make the dependencies on i915_drv.h explicit.

Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/39e99b765b8c1a05d001659c39686a661ac268e2.1732104170.git.jani.nikula@intel.com