msm: mdss: Add sysfs node for mdss to give runtime fps

Added sysfs node on mdss to get fps value at runtime.
The calculation is time based where in mdss driver
counts number of frames transferred for every 1 second.

Change-Id: I6f38298d066852d1a41cb90167aa41f594eec9c0
Signed-off-by: Raviteja Tamatam <travitej@codeaurora.org>
Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>