msm: mdss: expose MDP capabilities through sysfs on 8x10

Currently, MDP capabilities are exposed to the client via
a dedicated ioctl.  As more capabilities are added, it becomes
difficult to have to extend and maintain header file compatibility.
This change is to add a sysfs node which can be used to get the
MDP capabilities.

Change-Id: I9eb2e5546f708eb3f46c61f9abe4f25f23cd64aa
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>