drm: Simplify drm_for_each_legacy_plane arguments

No need to pass the planelist when everyone just uses
dev->mode_config.plane_list anyway.

I want to add a pile more of iterators with unified (obj, dev)
arguments. This is just prep.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>