drm/msm/sde: set minimum restriction on max_bw_high debugfs

This change sets a minimum value to max_bw_high. Input from
debugfs must be greater than or equal to the minimum or else
it will be set to the minimum value.

Change-Id: Id6c830d9665ec84d1eb5b41dee9274cad1596419
Signed-off-by: Samantha Tran <samtran@codeaurora.org>