drm/amd/display: Only flush delta from last command execution

[Why]
We're currently flushing commands that had been previously been
flushed or are currently being processed by the DMCUB when we don't
immediately wait for idle after command execution.

[How]
Avoiding reflushing the data by keeping track of the last wptr.

We'll treat this as the actual rptr by creating a copy of the inbox
and modifying the copy's rptr.

Reviewed-by: Eric Yang <Eric.Yang2@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>