drivers/video: fsl-diu-fb: clean up reset of primary display

Commit 4b5006ec ("shared DIU framebuffer support") added the ability to
retain the splash screen until the framebuffer is opened by user space.
Clean up this code to eliminate redundant writes to registers, and eliminate
the use of dummy area descriptor.

Signed-off-by: Timur Tabi <timur@freescale.com>