drm/omap: Remove cpu_is_omapXXXX usage in DMM

Removed usage of the cpu_is_omapXXXX in the DMM driver.  This is no
longer necessary as we can key off of the omap_dmm pointer that is
only non-NULL if the device has been probed successfully.

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