staging: drm/omap: Disable DMM debugfs for OMAP3

OMAP3 does not contain a DMM/Tiler block.  As such, we should not
be exposing any DMM debugfs entries on OMAP3 platforms.

Added inline helper function to verify existence of DMM.

Signed-off-by: Andy Gross <andy.gross@ti.com>
Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>