staging/fbtft: Fix backlight

staging/fbtft: Fix backlight

Commit b4a1ed0cd18b ("fbdev: make FB_BACKLIGHT a tristate") forgot to
update fbtft breaking its backlight support when FB_BACKLIGHT is a module.

Since FB_TFT selects FB_BACKLIGHT there's no need for this conditional
so just remove it and we're good.

Fixes: b4a1ed0cd18b ("fbdev: make FB_BACKLIGHT a tristate")
Cc: <stable@vger.kernel.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://lore.kernel.org/r/20211105204358.2991-1-noralf@tronnes.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>