[Why]
We read memory that we shouldn't be touching if the struct isn't
a full union dmub_rb_cmd.

[How]
Fix up all the callers and functions that take in the dmub_cmd_header
to use the dmub_rb_cmd instead.
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>