Fixes crash during reclocking.
Call Trace:
 pll_calc == NULL
 calc_pll
 calc_mclk
 nv50_pm_clocks_pre
 nouveau_pm_perflvl_set
 nouveau_pm_trigger
 nouveau_pm_profile_set
 nouveau_pm_set_perflvl
 dev_attr_store
 sysfs_write_file
 vfs_write
 sys_write
 system_call_fastpath
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>