drm/msm/dp: rename the enums for DP test bit depth

Rename the enums for test bit depth. This is needed to avoid
compilation errors once the SDE DP driver aligns closer to the
upstream DRM DP helper definitions.

Change-Id: I092d30b52a19955b64be3fcf574e34f6900c3474
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>