drm/tegra: Fix error handling

It is likely that checking 'gr3d->clk_secondary' instead of 'gr3d->clk'
is expected here.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>