disp: msm: dp: add support for diff values of pre-emp and swing levels

Update the DP controller and PHY programming to support different
values for pre-emp and voltage swing levels at different link rates.

Change-Id: I974c47cd1bf71b76dea4f270031a9cdfe320725e
Signed-off-by: Rajat Gupta <rajatgu@codeaurora.org>