clk: tegra: pll: Fix potential sleeping-while-atomic

Since the ->enable() callback is called with a spinlock held, we cannot
call potentially blocking functions such as clk_get_rate() or
clk_get_parent(), so use the unlocked versions instead.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
[rklein: Adapted from ChromeOS patch, removing pllu_enable cleanup as
it isn't present upstream]
Signed-off-by: Rhyland Klein <rklein@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>