OMAPDSS: DISPC: add dispc_mgr_get_framedone_irq

Add dispc_mgr_get_framedone_irq() which returns the irq number for
FRAMEDONE for the given channel.

Note that the function returns always 0 for DIGIT channel, even if OMAP4
does have FRAMEDONE_TV interrupt. The reason for this is that this
function is currently used only to track manual updates, and thus
FRAMEDONE_TV is not needed.

If there's need in the future to also get the FRAMEDONE_TV this needs
revisiting. However, as FRAMEDONE_TV doesn't exist on OMAP2/3, the use
of this function for that purpose needs some extra code to handle the
OMAP2/3 case.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>