msm: mdss: expose MDP capabilities through sysfs

User space applications currently rely on special private ioctl to query
capabilities of MDP driver. As more capabilities are added it becomes
more difficult to have to extend and maintain header file compatibility.
Provide a sysfs node which can be queried for MDP capabilities.

Change-Id: I59efb29bf465083a1ef65c0419226788b44085b1
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>
Signed-off-by: Shalabh Jain <shalabhj@codeaurora.org>