drm/imx: lift 64x64 pixel minimum framebuffer size requirement

There is no reason to limit framebuffer size to 64x64 pixels at a
minimum on creation. The actual scanout limitations (width >= 13 for the
base plane and height >= 2) are checked in atomic_check.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>