drm/amd/display: Initialize dmub_rb_cmd unions to 0

[Why]
Since dmub_rb_cmds are not initialized to 0,
the header is filled with invalid data.
This is causing issues on the fw side.

[How]
Initialize dmub_rb_cmd unions to 0.

Signed-off-by: Wyatt Wood <wyatt.wood@amd.com>
Reviewed-by: Josip Pavic <Josip.Pavic@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>