msm: mdss: allow assertive display on write back framebuffer

Reserve a mixer that supports AD to be used for write back and expose a
sysfs node for frame buffer devices which are able to support AD.

Change-Id: I0598a686d251444fc40bb8073ac50750036be7fe
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>
Signed-off-by: Ping Li <quicpingli@codeaurora.org>