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

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 BL by display's powerState.

Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Zhongwei Zhang <Zhongwei.Zhang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>