msm: mdss: fix vreg pre/post on/off wait duration

Commit be5fd56b ("msm:
mdss: use usleep_range instead of msleep in mdss util")
replaces the msleep with usleep_range but does not fix
the sleep duration. That leads to invalid wait time
after vreg pre/post on/off operation.

Change-Id: I47ae12eda393fd6367e7045a9bd91add3bb4a942
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>