omapfb: dispc: allow multiple external IRQ handlers

Previously, the only external (to dispc.c) IRQ handler was RFBI's frame
done handler.  dispc's IRQ framework was very dumb: you could only have
one handler, and the semantics of {request,free}_irq were odd, to say the
least.

The new framework allows multiple consumers to register arbitrary IRQ
masks.

Signed-off-by: Daniel Stone <daniel.stone@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Imre Deak <imre.deak@nokia.com>
Acked-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>