msm: mdss: Add debugfs support to change MDP max bandwidth

Based on various use-cases such as camera and flip, MDP bandwidth
limit can be changed. For debug purpose add support to change
these paramaters via debugfs. First parameter takes mode and
second paramter takes bandwidth limit for that particular mode.

E.g.  echo 2 1700000 > <debugfs>/mdp/perf/threshold_bw_limit

Change-Id: I98456f4f00223136628b2d2300b5785af386b134
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>