drm/{i915,xe}: Move intel_pch under display

drm/{i915,xe}: Move intel_pch under display

The only usage of the "PCH" infra is to detect which South Display
Engine we should be using. Move it under display so we can convert
all its callers towards intel_display struct later.

No functional or code change.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/041e3dee494aa15c22172360f2bdd9b15e4acb00.1744364975.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>