drm/i915: Save some lines of source code in workarounds

No functional or code size change - just notice we can compact the source
by re-using a single helper for adding workarounds.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/1548983324-15344-3-git-send-email-talha.nassar@intel.com