drm/tegra: Don't set a gamma table size

Tegra doesn't have any functions to set gamma tables, so this is
completely defunct.

Not nice to lie to userspace, so let's stop!

Cc: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>