OMAPDSS: Remove video SRAM support

OMAP SRAM can be used as video memory on OMAP1 and 2. However, there
usually is very little SRAM available, thus limiting its use, and no
board supported by the kernel currently uses it.

This patch removes the use of SRAM as video ram for the omapdss driver
to simplify memory handling.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>