disp: msm: sde: replace demura pipe index cap. with general pipe index

Generalize the demura pipe index capability on sde_plane objects to
instead provide the relative pipe index for all plane types. This
information can be used by clients to fully determine a plane type
and its position.

Change-Id: I597da0dab92c249b16934b9327353e4937635606
Signed-off-by: Christopher Braga <cbraga@codeaurora.org>