mod/dvfs/test: Fix uninitialised pointer issue in mod_dvfs unit test

Some unit tests in mod_dvfs_unit_test.c initialise local data
structures with the minimum information required to test DVFS
functions.

The local data structures sometimes have pointers in them that
depending on the particular test being run - should also be
initialised, but in some cases the unit test failed to do that.

This resulted in some pointers being initialised with whatever
random data was on the stack and depending on what ended in those
pointers, accessing data fields through them could end in a segfault.

The fix makes sure all pointers that need initialising are
properly set.

Signed-off-by: Francesco Rosati <francesco.rosati@arm.com>