udlfb: use spin_lock_irq instead of spin_lock_irqsave

udlfb: use spin_lock_irq instead of spin_lock_irqsave

spin_lock_irqsave and spin_unlock_irqrestore is inteded to be called from
a context where it is unknown if interrupts are enabled or disabled (such
as interrupt handlers). From a process context, we should call
spin_lock_irq and spin_unlock_irq, that avoids the costly pushf and popf
instructions.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>