drm/test: fix the gem shmem test to map the sg table.

The test here creates an sg table, but never maps it, when
we get to drm_gem_shmem_free, the helper tries to unmap and this
causes warnings on some platforms and debug kernels.

This also sets a 64-bit dma mask, as I see an swiotlb warning if I
stick with the default 32-bit one.

Fixes: 93032ae6 ("drm/test: add a test suite for GEM objects backed by shmem")
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Marco Pagani <marpagan@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240715083551.777807-1-airlied@gmail.com


Signed-off-by: Maxime Ripard <mripard@kernel.org>