The reference manual (for the i.MX25 in my case) states:

	This field must start at a location that enables a complete
	picture to be stored in a 4 Mbyte memory boundary (A [21:0]). A
	[31:22] has a fixed value for a picture’s image.

Check this condition for user-supplied framebuffer values and enforce it
for driver-allocated buffers.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>