drm/i915: Pass the dma_addr_t array as const to rotate_pages()

rotate_pages() doesn't modify the passed in dma addresses, so make
them const.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1452777736-4909-4-git-send-email-ville.syrjala@linux.intel.com