OMAP: DSS2: DSI: enable interface for omap4

By default, the DSI is not getting enabled for omap4.
OMAP2PLUS does not catch this issue since it has ARCH_OMAP3.

The issue is only seen when using defconfig with ARCH_OMAP4 only.

Signed-off-by: Vikram Pandita <vikram.pandita@ti.com>
Cc: Archit Taneja <archit@ti.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>