subsurface: Add a function for getting parent of sub-surface

subsurface: Add a function for getting parent of sub-surface

To get a parent of sub-surface without accessing particular sub-surface
data, this adds e_comp_wl_subsurface_panret_get() function. This should
also help with readability.

As the name implies, this should be called with a E_Client associated
with sub-surface instance, but it will be okay to be called with
not sub-surface instance. In that case, it will just return NULL.

Change-Id: I08f340424b46fdb54141fa41747ba55ad2ae6875