drm/i915: Identify Cometlake platform

Cometlake is a small refresh of Coffeelake, but since we have found out a
difference in the plaforms, we need to identify them as separate platforms.

Since we previously took Coffeelake/Cometlake as identical, update all
IS_COFFEELAKE() to also include IS_COMETLAKE().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200602140541.5481-1-chris@chris-wilson.co.uk