drm/amd/display: Add visualconfirm module parameter

[Why]

Being able to configure visual confirm at boot or in cmdline is helpful
when debugging.

[How]

Add a module parameter to configure DC visual confirm, which works the
same way as the equivalent debugfs entry.

Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>