drm/tegra: Fix comparison operator for buffer size

Here we are checking for the buffer length, not an offset for writing
to, so using > is correct. The current code incorrectly rejects a
command buffer ending at the memory buffer's end.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>