OMAPDSS: DISPC: Add IRQ enable/status helpers

DISPC irqs need to be handled from the compat layer and also in the
future by the omapdrm. To make this possible, this patchs adds a set of
helper functions, so that the irqs can be managed without direct
register reads/writes.

The following functions are added, and all the current direct reg
reads/writes are changed to use these.

u32 dispc_read_irqstatus(void);
void dispc_clear_irqstatus(u32 mask);
u32 dispc_read_irqenable(void);
void dispc_write_irqenable(u32 mask);

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