drm/amd/display: Add ability for HWSS to prepare the DDC before use

drm/amd/display: Add ability for HWSS to prepare the DDC before use

Make it possible to add a HWSS function to prepare the DDC
before trying to use it. This is going to be necessary for
external DP bridge encoders.

This commit just adds the function to common DC code.
The actual implementation of this function for DCE is
done in a subsequent commit.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>