drm/i915: remove CNL leftover

We left the definition IS_CANNONLAKE() macro while removing it from the
tree due to having to merge the changes in different branches. Now that
everything is back in sync and nobody is using IS_CANNONLAKE(), we can
safely ditch it.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211021181847.1543341-1-lucas.demarchi@intel.com