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>