OMAP2,3: DSS2: Get DSS IRQ from platform device

DSS IRQ number can be obtained from platform_get_irq().  This API in turn
picks the right IRQ number belonging to HW IP from the hwmod database.
So hardcoding of IRQ number could be removed.

This IRQ is stored in dss_irq as part of dss structure, and freed it in
dss_exit().

Reviewed-by: Paul Walmsley <paul@pwsan.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Tested-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Senthilvadivu Guruswamy <svadivu@ti.com>
Signed-off-by: Sumit Semwal <sumit.semwal@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>