clk: tegra: Set the clock parent of gr2d/gr3d to pll_c2

pll_m will be the parent of gr2d/gr3d if we don't do this.
And because pll_m runs at a high rate so gr2d/gr3d will be
unstable. So change the parent of them to pll_c2.

Signed-off-by: Mark Zhang <markz@nvidia.com>
Acked-By: Peter De Schrijver <pdeschrijver@nvidia.com>