backlight/video: Use Platform getter/setter functions

Use getter and setter functions, for platform_device structures and a
spi_device structure.

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Lee Jones <lee.jones@linaro.org>