clk: ti: autoidle: add checks against NULL pointer reference

The clk pointer passed to omap2_clk_(deny|allow)_idle can be NULL, so
add checks for this.

Reported-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>
Link: https://lore.kernel.org/r/20200907082600.454-2-t-kristo@ti.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>