drm/amd/display: Print seamless boot message in mark_seamless_boot_stream

[WHAT & HOW]
Add a message so users know the stream will be used for seamless boot.

Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>