drm/i915: update the firmware download URL

The old URL works but gives 301 Moved Permanently. Update.

Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1489574966-27200-1-git-send-email-jani.nikula@intel.com