drm/i915: use BUILD_BUG_ON to ensure platform name has been set up

Leave the runtime check in place in case the platform variable itself
comes from bogus sources.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1488280303-9323-1-git-send-email-jani.nikula@intel.com