drivers/video: fsl-diu-fb: fix compilation warning

Fix this compilation warning in the Freescale DIU framebuffer driver:

warning: 'dummy_ad_addr' may be used uninitialized in this function

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