msm: display: Proper handling of prepare and unprepare clocks

This change handles prepare and unprepare clocks properly to
fix clock mismatches.

Change-Id: I09986aa1f0e1a5be2e07655a1db0d23760dd0725
Signed-off-by: Sangani Suryanarayana Raju <csuryan@codeaurora.org>