drm: omapdrm: Handle FIFO underflow IRQs internally

As the FIFO underflow IRQ handler just prints an error message to the
kernel log, simplify the code by not registering one IRQ handler per
plane but print the messages directly from the main IRQ handler.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>