OMAPFB: clear framebuffers with CPU

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.

This patch implements clearing of the framebuffer in omapfb, using
cfb_fillrect() to do the actual clear.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>