video: fbdev: pxafb: loosen the platform data bond

In order to prepare the transition to a mixed platform data and
device-tree initialization, remove all the platform data references all
over the driver.

Copy the platform data into the internal structure of the pxafb, and
only use this afterward.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>