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

Cc: Sudip Mukherjee <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Laurent Pinchart <[email protected]>
Cc: Jingoo Han <[email protected]>
Cc: Suresh Siddha <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Dave Airlie <[email protected]>
Cc: Antonino Daplas <[email protected]>
Cc: Jean-Christophe Plagniol-Villard <[email protected]>
Cc: Tomi Valkeinen <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Luis R. Rodriguez <[email protected]>
Reviewed-by: Dave Airlie <[email protected]>
Signed-off-by: Tomi Valkeinen <[email protected]>

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