[WHY]
Flush command sent to DMCUB spends more time for execution on
a dGPU than on an APU. This causes cursor lag when using high
refresh rate mouses.

[HOW]
1. Change the DMCUB mailbox memory location from FB to inbox.
2. Only change windows memory to inbox.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Lewis Huang <lewis.huang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>