drm/amd/display: fix dpms_off issue when disabling bios mode

[Why]
disable_vbios_mode_if_required() will set dpms_off to false during boot
when pixel clk dismatches with driver requires. This will cause extra
backlight on and off if OS call 2 times setmode.

[How]
Set dpms_off to true to keep power_off and let OS control backlight by
display's powerState.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Zhongwei <Zhongwei.Zhang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>