msm: clock-mdss: Move DSI PLL enable seq to prepare ops

DSI PLL enable sequences involve significant amount of
hardware recommended delays. As such, these sequences should
not be called while holding spinlocks (such as from within
clock enable API) as they may result in delayed signalling
of critical interrupts. To fix this, move the PLL enable
sequences to the clock prepare API.

Change-Id: I4d0dc645e57b425c70d8d146fab1babf6c67ca6d
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>