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

Only one reference to CNL that is not needed, but code is the same for
DISPLAY_VER >= 11, so leave the code around and just remove the special
case for CNL.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210728215946.1573015-8-lucas.demarchi@intel.com