drm/i915: mark userptr objects as ALLOC_USER

These are userspace objects, so mark them as such. In a later patch it's
useful to determine how paranoid we need to be when managing cache
flushes. In theory no functional changes.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211018174508.2137279-2-matthew.auld@intel.com