msm: mdss: dp: add support for DP PHY register offset property

Add a DT property to specify DP PHY register offset for display
port. There is a register offset difference for a few DP PHY
registers between msm8998 and SDM660 and hence this is needed.
Add changes to program DP PHY registers by accounting for this
register offset difference wherever applicable.

Change-Id: I515432830ae6c3fa3223f0c97af7b0a3965afc40
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>