drm/amd/display: Bug in dce_is_panel_backlight_on()

[Why]
dce_is_panel_backlight_on() will return wrong value if
LVTMA_BLON_OVRD is 0

[How]
When LVTMA_BLON_OVRD is 0, read
LVTMA_PWRSEQ_TARGET_STATE instead

Signed-off-by: Peikang Zhang <peikang.zhang@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>