clk: qcom: mdss: split the DSI PLL driver based on PLL mode

Re-organize the DSI PLL driver code and split it based on
the DSI PLL HPM/LPM mode. Add a common PLL util file to use
the APIs which are common for both PLLs. Update the DSI PLL
enable sequence with the recommended settings for LPM mode.

Change-Id: I3f86554522e16579d5c2eccab976136c7afb0dd2
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>