drm/msm/dp: add support for DisplayPort TPG for pixel data

Add change to support test pattern generation for pixel data using
DP BIST pattern. Add a debugfs node to enable/disable TPG runtime
for debug purposes.

Change-Id: Idf62bf7636a1e678bbec5068ed7f60112a8ac458
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>