drm/msm/sde: add sysfs node in dpu driver to provide fps

Add sysfs node on crtc to get fps value at runtime.
The algorithm will return last committed frame count in
the configured periodicity timeline. The calculation is
time based where in dpu driver counts number of frames
for the given periodicity value, otherwise it counts the
number of frames transferred for last one second.

Below are the commands to give in shell:

Command to set periodicity:
	echo value > /sys/class/drm/sde-crtc-*/fps_periodicity_ms

Command to get periodicity:
	cat /sys/class/drm/sde-crtc-*/fps_periodicity_ms

Command to get the measured fps:
	cat /sys/class/drm/sde-crtc-*/measured_fps

Replace * with 0 for primary, 1 for dp and 2 for wfd.

Change-Id: I8fe2bcb5738c321f3472caa6c370d0713e27f237
Signed-off-by: Shubhashree Dhar <dhar@codeaurora.org>