The option isn't FlatpanelDisplay. The option specially mentioned in the driver docs is:
Option "FlatPanelProperties" "string"
Requests particular properties of any connected flat
panels as a comma-separated list of property=value pairs.
Currently, the only two available properties are 'Scaling'
and 'Dithering'. The possible values for 'Scaling' are:
'default' (the driver will use whatever scaling state
is current), 'native' (the driver will use the flat
panel's scaler, if it has one), 'scaled' (the driver
will use the NVIDIA scaler, if possible), 'centered'
(the driver will center the image, if possible),
and 'aspect-scaled' (the driver will scale with the
NVIDIA scaler, but keep the aspect ratio correct).
The possible values for 'Dithering' are: 'default'
(the driver will decide when to dither), 'enabled' (the
driver will always dither when possible), and 'disabled'
(the driver will never dither). If any property is not
specified, it's value shall be 'default'. An example
properties string might look like:
"Scaling = centered, Dithering = enabled"