drm/i915/cfl: Remove alpha support protection.

We now have Coffee Lake on our CI systems.

Coffee Lake is at this point in same stage as Kaby Lake.

And it seems that we don't have any risk of bad blank
screens or anything like that. So let's remove the protection.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170907230632.25650-1-rodrigo.vivi@intel.com