drm/i915/cnl: Gen10 render context size.

No change on render context size is required for Gen10.

So this patch doesn't change the default behaviour,
but only avoid the missing_case message.

Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Link: http://patchwork.freedesktop.org/patch/msgid/1499375184-5725-1-git-send-email-rodrigo.vivi@intel.com