drm/i915: Drop dma_buf->k(un)map

No in-tree users left.

Aside, I think mock_dmabuf would be a nice addition to drm
mock/selftest helpers (we have some already), with an
EXPORT_SYMBOL_FOR_TESTS_ONLY.

Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: "Christian König" <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191118103536.17675-7-daniel.vetter@ffwll.ch