drm/amd/display: Create dpcd and i2c packing functions

[Why]
We need to read and write specific i2c and dpcd messages.

[How]
Created static functions for packing the dpcd and i2c messages for hdcp.

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