drivers/video: fsl-diu-fb: streamline enabling of interrupts

Remove functions request_irq_local() and free_irq_local(), and merge
their code into the functions that handle opening and closing the
framebuffer.  We also improve the way that interrupts are enabled.

Also implement a work-around for broken U-Boots that leave the DIU
interrupts enabled.

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