Do null check properly for dvfs->get_avail_governor().
Change-Id: I70edddcf552ff677a25611c948ad9e8a2572025a
Fixes: 9f76c33751ba ("halapi: power: Add exec_func helper to simplify the function call") Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>