ARM: OMAP2+: display: Create omapdrm device inside omap_display_init

Move omapdrm device creation inside the omap_display_init so that we can
correctly create the device based on the presence of omapdss within the
platform.

For example, on a kernel image supporting multiple platforms, omap_init_drm
will create a omapdrm platform device on a AM33xx platform even though it
doesn't have a DSS block.

Originally worked on by Andy Gross.

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>