disp: msm: sde: add tracking for if perf_mode is changed

Perf tuning mode can be updated through debugfs and this should
be used to reflect the core clock rate.

Change-Id: I313d079f0b8013f43f4b293c6400f34eaf56b6d2
Signed-off-by: Nilaan Gunabalachandran <ngunabal@codeaurora.org>