drm/vgem: Use PAGE_KERNEL in place of x86-specific PAGE_KERNEL_IO

Since PAGE_KERNEL_IO is specific to x86 and equivalent to PAGE_KERNEL
for our wrapping with pgprot_writecombine(), just use the common define.

   drivers/gpu/drm/vgem/vgem_drv.c: In function 'vgem_prime_vmap':
>> drivers/gpu/drm/vgem/vgem_drv.c:238:53: error: 'PAGE_KERNEL_IO' undeclared (first use in this function)
     addr = vmap(pages, n_pages, 0, pgprot_writecombine(PAGE_KERNEL_IO));

Reported-by: 0day
Fixes: e6f15b76 ("drm/vgem: Enable dmabuf interface for export")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1468325090-27966-1-git-send-email-chris@chris-wilson.co.uk