drivers/video/fbdev: use ioremap_wc/wt() instead of __ioremap()

_PAGE_NO_CACHE is a platform specific flag. In addition, this flag
is misleading because one would think it requests a noncached page
whereas a noncached page is _PAGE_NO_CACHE | _PAGE_GUARDED

_PAGE_NO_CACHE alone means write combined noncached page, so lets
use ioremap_wc() instead.

_PAGE_WRITETHRU is also platform specific flag. Use ioremap_wt()
instead.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>