drm/amd/display: workaround for oled eDP not lighting up on DCN401

[Why]
Currently there's an issue on DCN401 that prevents oled eDP panels from
being lit up that is still under investigation. To unblock dev work
while investigating, we can work around the issue by skipping toggling
the enablement of the backlight.

[How]
 - new debug bit that will skip touching backlight enable DPCD for oled

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>