drm/amd/display: Turn off internal backlight when plugging external monitor

[why]
For VG, we want to turn off power/backlight of the intenral panel when
plugging in external monitor and going to "external monitor only" mode.

[how]
For turning off power of the internal panel, ignore the config flag whic
bypasses power sequencing for eDP panels.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Sung Joon Kim <sungkim@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>