drm/tegra: Map cmdbuf once for reloc processing

A few reasons to drop kmap:

- For native objects all we do is look at obj->vaddr anyway, so might
  as well not call functions for every page.

- Reloc-processing on dma-buf is ... questionable.

- Plus most dma-buf that bother kernel cpu mmaps give you at least
  vmap, much less kmaps. And all the ones relevant for arm-soc are
  again doing a obj->vaddr game anyway, there's no real kmap going on
  on arm it seems.

Plus this seems to be the only real in-tree user of dma_buf_kmap, and
I'd like to get rid of that.

Reviewed-by: Thierry Reding <treding@nvidia.com>
Tested-by: Thierry Reding <treding@nvidia.com>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: linux-tegra@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20191118103536.17675-2-daniel.vetter@ffwll.ch