drm/i915/display: convert intel_cpu_transcoder_mode_valid() to intel_display

Going forward, struct intel_display is the main display device data
pointer. Convert the intel_cpu_transcoder_mode_valid()() helper to
struct intel_display, allowing further conversions elsewhere.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f9246a00a2e7aabaffb86f863915a4307e1fd3f8.1739378095.git.jani.nikula@intel.com