drm/amd/display: Refactor mark_seamless_boot_stream()

mark_seamless_boot_stream() can be called multiple times to run
the more expensive checks in dc_validate_boot_timing().

Refactor the function so that if those have already passed once
the function isn't called again.

Also add a message the first time that they have passed to let
the user know the stream will be used for seamless boot.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Link: https://lore.kernel.org/r/20250120194903.1048811-4-superm1@kernel.org


Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>