msm:mdss: Add performance tuning parameters in debugfs

Adding params in debugfs to tune the performance of the driver
at runtime from debugfs. These params are min_bus_vote,
clk_factor, ab and ib factor, bw thresholds etc. User can tweak
these parameters from debugfs to help in debugging the issues.

Change-Id: Ie134eba35bf81ecc1524fba29bae902870673bd0
Signed-off-by: Vineet Bajaj <vbajaj@codeaurora.org>