[arm/tegra] Check null pointer in suspend.c

[arm/tegra] Check null pointer in suspend.c

In tegra_suspend_enter, we need to check mask() and unmask() for null
before calling them, since there is no guarantee irq chips other than
tegra will implement these functions.

Change-Id: Ia6cb2c234983722bbe1202ec84eaceaf22a13450
Reviewed-on: http://git-master/r/20099
Tested-by: Thomas Cherry <tcherry@nvidia.com>
Reviewed-by: Sachin Nikam <snikam@nvidia.com>
Reviewed-by: Bharat Nihalani <bnihalani@nvidia.com>