video: fbdev: geode gxfb: use ioremap_wc() for framebuffer

The driver doesn't use mtrr_add() or arch_phys_wc_add() but
since we know the framebuffer is isolated already on an
ioremap() we can take advantage of write combining for
performance where possible.

In this case there are a few motivations for this:

a) Take advantage of PAT when available

b) Help with the goal of eventually using _PAGE_CACHE_UC over
   _PAGE_CACHE_UC_MINUS on x86 on ioremap_nocache() (see commit
   de33c442 titled "x86 PAT: fix performance drop for glx,
   use UC minus for ioremap(), ioremap_nocache() and
   pci_mmap_page_range()")

Cc: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Jingoo Han <jg1.han@samsung.com>
Cc: Suresh Siddha <sbsiddha@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Juergen Gross <jgross@suse.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Antonino Daplas <adaplas@gmail.com>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: linux-geode@lists.infradead.org
Cc: linux-fbdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/video/fbdev/geode/gxfb_core.c

@@ -263,7 +263,8 @@ static int gxfb_map_video_memory(struct fb_info *info, struct pci_dev *dev)
 
 	info->fix.smem_start = pci_resource_start(dev, 0);
 	info->fix.smem_len = vram ? vram : gx_frame_buffer_size();
-	info->screen_base = ioremap(info->fix.smem_start, info->fix.smem_len);
+	info->screen_base = ioremap_wc(info->fix.smem_start,
+				       info->fix.smem_len);
 	if (!info->screen_base)
 		return -ENOMEM;