drm: add helper to clflush a virtual address range

Useful when the page is already mapped to copy date in/out.

For -stable because the next patch (fixing phys obj pwrite) needs this
little helper function.

Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>