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

Remove DMC firmware for CNL.

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/20210728215946.1573015-9-lucas.demarchi@intel.com