drm/amd/display: take dpms_off into account for edp turn off logic

[why]
Previously we incorrectly skipped backlight control when stream is
present but dpms_off = true. This causes backlight to remain on in
the we boot up or resume into a external display only configuration
where VBIOS posted on the eDP.

[How]
Add dpms_off into the condition for edp need to turn off.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>