drm/omap: fbdev: use 'screen_buffer' field

Fix sparse warning:

drivers/gpu/drm/omapdrm/omap_fbdev.c:191:26: warning: incorrect type in assignment (different address spaces)
drivers/gpu/drm/omapdrm/omap_fbdev.c:191:26:    expected char [noderef] <asn:2>*screen_base
drivers/gpu/drm/omapdrm/omap_fbdev.c:191:26:    got void *

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