drm/i915: Use HAS_CSR instead of gen number on DMC load.

Since we have HAS_CSR tied to the platform definition
let's use this instead of checking per platform.

One less thing to worry when adding support to new platforms.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Animesh <Manna&lt;animesh.manna@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1497047175-27250-14-git-send-email-rodrigo.vivi@intel.com