drm/i915: Avoid memory leak with more than 16 workarounds on a list

I forgot to free the old list when growing past 16 entries.

Luckily, as much as I checked, none of the current platforms has more than
16 workarounds on a single list.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 452420d2 ("drm/i915: Fuse per-context workaround handling with the common framework")
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201113132510.2298483-1-tvrtko.ursulin@linux.intel.com