OMAP: DSS2: Remove DSS_IRQSTATUS

DSS_IRQSTATUS register is available only on OMAP3, and reading it in
dss_dump_registers() on OMAP4 seems to cause a crash.

The register is not used for anything, and displaying its value is of no
use, so let's just remove it altogether.

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