sunxi-disp: Correct fex file key for sun7i lcd backlight intensity

When support for setting the initial lcd backlight intensity through the fex
file was added to the linux-sunxi tree with commit b243bae3, it was using
lcd#_bright as key.

This is correct for sun4i/sun5i, but the allwinner sources for sun7i are using
lcd#_backlight, they are also using lcd#_bright but as a value for brightness
adjustment by the display enhancement engine. To make things worse, lcd#_bright
has a scale of 0 - 100, so typically has a value of 50, where as lcd#_backlight
has a range of 0-255. This means that for sun7i fex-files which use the offical
lcd#_backlight and lcd#_bright, we take the lcd#_bright value of 50 and
interprete it on a 0-255 scale, we effectively turn the backlight off!

This patch fixes this by properly checking for lcd#_backlight as backlight
intensity setting on sun7i.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>