drm/msm/dp: add the support to read DP test/link parameters

Add the support to read the DP test and link parameters using
an unified API in DP link driver. Cleanup the DP link and test
parameter structures for better readability.

Change-Id: I5fcac239d98ec725339eded0156c1a287127226b
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>