disp: msm: limit display brightness max cooling device level

Based on panel hardware support, display brightness levels can
be very high value. This high value display brightness cooling
device levels can cause exceeding PAGE_SIZE for cooling device stat
buffer. It leads to buffer failure for cooling device stat feature.

Limit display panel mitigation level max to 255. If hardware
supports more than 255, then scale brightness levels fit
into above limit.

Change-Id: Ieeee4ff2aa5cd884819b30b4fd9839e48ac4d804
Signed-off-by: Manaf Meethalavalappu Pallikunhi <manafm@codeaurora.org>