drm/amd/display: Skip dmub memory flush when not needed

[WHY&HOW]
Readback is only necessary when loaded via CPU.

Reviewed-by: Chris Park <chris.park@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>