drm/amd/display: Add front end for dp debugfs files

As part of hardware certification, read-write access to
the link rate, lane count, voltage swing, pre-emphasis,
and PHY test pattern of DP connectors is required.  This commit
adds debugfs files that will correspond to these values.
The file operations are not yet implemented: currently
writing or reading them does nothing.

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>