drm/i915: finish removal of CNL

With all the users removed, finish removing the CNL platform definitions.
We will leave the PCI IDs around as those are exposed to userspace.
Even if mesa doesn't support CNL anymore, let's avoid build breakages
due to changing the headers.

Also, due to drm/i915/gt still using IS_CANNONLAKE() let's just redefine
it instead of removing.

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/20210728215946.1573015-26-lucas.demarchi@intel.com