drm/amd: Add a module parameter for seamless boot

The module parameter can be used to test more easily enabling seamless
boot support on additional ASICs.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>