drivers/video: fsl-diu-fb: set correct framebuffer flags

The DIU uses system RAM for the framebuffer, so FBINFO_VIRTFB should be set.

Since the framebuffer is in system RAM, it can be read from more quickly
than written to, so FBINFO_READS_FAST should be set.

We can also set FBINFO_PARTIAL_PAN_OK for the same reasons.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>