ARM: OMAP2+: omap2plus_defconfig: enable CMA allocator

The OMAP framebuffer driver now uses the standard dma allocator
instead of the (now removed) omap specific vram allocator.

Enable the Contiguous Memory Allocator by default to allow large
dma memory buffers allocation.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>