FB: fix unsafe use of disable_irq() in mx3fb.c

mx3fb.c calls disable_irq() from a DMA callback, i.e., in an IRQ-handler
context, which has always been unsafe, and became deadly after the merge of
threaded interrupt handler support. Use disable_irq_nosync() instead.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>