drm/amd/display: Removing unused code from dmub_cmd.h

Removing code that is not used at the moment.

Signed-off-by: Anson Jacob <Anson.Jacob@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>