drm/msm/dsi: Add a PHY op that initializes version specific stuff

Create an init() op for dsi_phy which sets up things specific to
a given DSI PHY.

The dsi_phy driver probe expects every DSI version to get a
"dsi_phy_regulator" mmio base. This isn't the case for 8x96.
Creating an init() op will allow us to accommodate such
differences.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>