[Why]
There's a data race that can occur between when we update the
inbox write pointer vs when the memory for the command actually gets
flushed from the map to the framebuffer.

DMCUB can read stale or partially invalid data when this race occurs.

[How]
Before updating the write pointer we can read back all pending commands
to ensure that we stall for the writes to be flushed to framebuffer.

We don't need to worry about choosing HDP vs VM flush with this
mechanism.

Drop the dmub_srv_cmd_submit() while we're updating this to work
correctly since nothing was actually using this API and the caller
should be explicit about the API flow here - by doing this on execute
we can give some extra time for the flush to finish while
preparing other commands.

We should try to avoid writing single commands
because of this overhead.
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>