msm: mdss: update sysfs node with max mixer width supported

Update the capabilities sysfs node with the max mixer width
supported to have userspace read and add checks for pipe
configurations.

Change-Id: I1431b8d06538b68d0dab4ac1698bd2a93331ad1c
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>