drm/msm/sde: add debugfs node in dpu driver to give fps

Add debugfs node on crtc to get fps value at runtime.
The calculation is time based where in dpu driver
counts number of frames transferred for every 1 second.

Change-Id: Ic6ba615cc0b394fd09f5953a7d26c9af8be3559d
Signed-off-by: Shubhashree Dhar <dhar@codeaurora.org>