OMAPDSS: VRAM: Remove clearing with sDMA

Currently vram.c clears the allocated memory automatically using OMAP
system DMA. In an effort to reduce OMAP dependencies, we'll do the
memory clear with CPU from now on.

The previous patch implemented memory clear in the omapfb driver, and
this patch removes the now obsolete clear functionality from vram.c.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Russell King - ARM Linux <linux@arm.linux.org.uk>