drm/msm/dsi-staging: fix potential out of bounds memory accesses

Use correct indexing when using the contianer_of operation to
get the DSI clock manager handle to avoid potential out of
bounds memory accesses.

Change-Id: I8804c5ff2bb602527f84749116e1408513e22759
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>