drm/msm/dsi-staging: separate link HS and LP clocks

Separate DSI high speed (HS) clocks from low power (LP) clock, so that
we can have better control in enabling and disabling these clocks
independently. Some DSI hardware versions need LP clocks to be turned on
before programming DSI PHY.

Change-Id: I05efde2bb2a7f737e2f952554b41e3ab74e990e5
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>