drm/i915: Relocate intel_crtc_for_plane()

Move intel_crtc_for_plane() next to its only user. No one
else should ever use this.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211203112029.1057-2-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>