drm/i915/chv: Fix error path in GPU freq helpers

Atm we wouldn't catch these errors or on the error path we would end up
with a division-by-zero, fix this up.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: David Weinehall <david.weinehall@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1454071949-24677-2-git-send-email-imre.deak@intel.com