drm/tegra: Fix vmapping of prime buffers

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>