drm/i915: Don't leak the capture list items

When we recently converted the capture code to use vma snapshots,
we forgot to free the struct i915_capture_list list items after use.

Fix that by bringing back a kfree.

Fixes: ff20afc4 ("drm/i915: Update error capture code to avoid using the current vma state")
Cc: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211209141304.393479-1-thomas.hellstrom@linux.intel.com