drm/i915/selftests: Prepare mman testcases for obj->mm.lock removal.

Ensure we hold the lock around put_pages, and use the unlocked wrappers
for pinning pages and mappings.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210323155059.628690-48-maarten.lankhorst@linux.intel.com