drm/i915/display: remove explicit CNL handling from intel_crtc.c

No need for special CNL handling as there is no real platform with that
configuration.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210728215946.1573015-6-lucas.demarchi@intel.com