drm/i915: Don't warn if the workaround list is empty.

It's not an error for the workaround list to be empty if no
workarounds are needed.  This will avoid spamming the logs
unnecessarily on Gen6 after the workaround list is hooked up on
pre-Gen8 hardware by the following commits.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>