msm: mdss: add support for dedicated hardware cursor

Hardware cursor within mdss from mdss v1.5 onwards is
programmed within the SSPP interface and is no longer
a separate dedicated stage in the layer mixer. Cursor
needs to be staged onto the existing layer mixer stages
and always at the top level available. From this version
onwards, cursors are expected to be programmed through
the overlay interface by the user.

Change-Id: Ia225dd92079e0153e9677ff035cf8531e62d5554
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>