The code assumes that Tegra GEM is permanently vmapped, which is not
true for the scattered buffers. After converting Tegra video decoder
driver to V4L API, we're now getting a BUG_ON from dma-buf core on playing
video using libvdpau-tegra on T30+ because tegra_gem_prime_vmap() sets
vaddr to NULL. Older pre-V4L video decoder driver wasn't vmapping dma-bufs.
Fix it by actually vmapping the exported GEMs.

Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>