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>

drivers/gpu/drm/omapdrm/omap_fbdev.c

@@ -188,7 +188,7 @@ static int omap_fbdev_create(struct drm_fb_helper *helper,
 
 	dev->mode_config.fb_base = dma_addr;
 
-	fbi->screen_base = omap_gem_vaddr(fbdev->bo);
+	fbi->screen_buffer = omap_gem_vaddr(fbdev->bo);
 	fbi->screen_size = fbdev->bo->size;
 	fbi->fix.smem_start = dma_addr;
 	fbi->fix.smem_len = fbdev->bo->size;