ARM: OMAP2+: drm: Don't build device for DMM

DMM exists on omap4+ platforms, these platforms are always expected to boot with
DT. Remove the current method of searching the dmm hwmod and building an
omap_device for dmm.

For OMAP4, the address and irq data for DMM hwmod(along with other blocks) were
removed, so the current method fails in the dmm driver's probe anyway.

The addition of DMM nodes in DT will ensure that a DMM device is built
correctly.

Cc: Andy Gross <andygro@gmail.com>
Signed-off-by: Archit Taneja <archit@ti.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>