drm/i915: Trim unused workaround list entries

The new workaround list allocator grows the list in chunks so will end up
with some unused space. Trim it when the initialization phase is done to
free up a tiny bit of slab.

v2:
 * Simplify with kmemdup. (Chris Wilson)

v3:
 * Refactor for __size removal.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181203125014.3219-8-tvrtko.ursulin@linux.intel.com