msm: mdss: cleanup dsi clock handling

Currently the DSI driver uses a two level reference counting for
the clocks used by the driver so as to ensure that the clocks are
correctly configured for dual-dsi use cases. However, this is not
necessary as the same can be achieved by using a single reference
count. Clean up the code to remove the sub-level ref counts and
add necessary error handling.

Change-Id: I6f1c1bfe83322dc0e1da3d757b6ca88cd629663e
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>