drm/msm/dsi-staging: use usleep_range instead of msleep in dsi enable

Use usleep_range api instead of msleep in dsi enable path to
improve accuracy, which improves bootup time.

Change-Id: I5d00d666bfacddea58b824267eb0eb39b5b2641c
Signed-off-by: Vara Reddy <varar@codeaurora.org>