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

The only real platform with DISPLAY_VER == 10 is GLK. We don't need to
handle CNL explicitly in intel_ddi.c.

Remove code and rename functions/macros accordingly to use ICL prefix.
There's one leftover reference to cnl that comes from the struct
intel_ddi_buf_trans. This will be renamed later when we get rid of the
additional CNL tables.

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/20210728215946.1573015-7-lucas.demarchi@intel.com