drm/msm/dsi-staging: use usleep for wait during command transfer to panel

Use usleep instead of msleep which will ensure the delay
needed during command transfer to panel.

Change-Id: Ia3f53a2f9b649cc5db629bb1a735fc99bbf9149d
Signed-off-by: Shashank Babu Chinta Venkata <sbchin@codeaurora.org>