Fix i915's CI build after the removal of the dmabuf->kmap interface that
left the mock routines intact.

In file included from drivers/gpu/drm/i915/i915_gem_dmabuf.c:335:0:
drivers/gpu/drm/i915/selftests/mock_dmabuf.c:104:13: error: ‘mock_dmabuf_kunmap_atomic’ defined but not used [-Werror=unused-function]
 static void mock_dmabuf_kunmap_atomic(struct dma_buf *dma_buf, unsigned long page_num, void *addr)
drivers/gpu/drm/i915/selftests/mock_dmabuf.c:97:14: error: ‘mock_dmabuf_kmap_atomic’ defined but not used [-Werror=unused-function]
 static void *mock_dmabuf_kmap_atomic(struct dma_buf *dma_buf, unsigned long page_num)

Fixes: f664a526 ("dma-buf: remove kmap_atomic interface")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Christian König <christian.koenig@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180620162152.1158-1-chris@chris-wilson.co.ukReviewed-by: Christian König <christian.koenig@amd.com>