drm/msm/dsi-staging: check clk handles before clk unprepare

Validate clk handles before clk unprepare. This would avoid
clk unprepare on uninitialized clk handles.

Change-Id: I76896effb7d5d78e96a6fc53e3082faf6c0839c4
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>