video: fbdev: atmel_lcdfb: 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
de33c442e titled "x86 PAT: fix performance drop for glx,
use UC minus for ioremap(), ioremap_nocache() and
pci_mmap_page_range()")

Cc: Nicolas Ferre <[e-mail]>
Cc: Suresh Siddha <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Juergen Gross <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Dave Airlie <[e-mail]>
Cc: Antonino Daplas <[e-mail]>
Cc: Jean-Christophe Plagniol-Villard <[e-mail]>
Cc: Tomi Valkeinen <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Luis R. Rodriguez <[e-mail]>
Acked-by: Nicolas Ferre <[e-mail]>
Reviewed-by: Dave Airlie <[e-mail]>
Signed-off-by: Tomi Valkeinen <[e-mail]>

drivers/video/fbdev/atmel_lcdfb.c

···
 			goto stop_clk;
 		}
 
-		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) {
 			ret = -ENOMEM;
 			goto release_intmem;