msm: mdss: Fix order of DSI clocks operations

For DSI branch clocks, set rate should be called
before calling the prepare and enable ops. This is
the recommended order for clock operations and is
required before moving the DSI pll enable sequences
to the prepare op.

Change-Id: I5f53f424a6665e2ce41076734eb6379596fa1d00
CRs-Fixed: 513361
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>