drm/amd/display: Don't check seamless boot in power down HW by timeout

[Why]
power_down_on_boot is designed to power down HW when set mode is not
called before timeout. It can happen in headless system or booting with
the display is output by non-AMD GPU only.
The function will be executed only if it's not seamless boot. So in
seamless boot, the HW is still on.
It's not necessary to check this since there's no display data in both
cases.

[How]
Remove seamless boot checking in power_down_on_boot.

Signed-off-by: John Wu <john.wu@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>