drm/i915: Remove dma_buf_kmap selftest

It's the only user left in the entire kernel for dma_buf_kmap/_kunmap.
Delete it, before we start garbage-collecting the various
implementations.

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: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Dave Airlie <airlied@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191118103536.17675-4-daniel.vetter@ffwll.ch