drm/i915/vlv: fix RC6 residency time calculation

The divider value to convert from CZ clock rate to ms needs a +1
adjustment on VLV just like on CHV. This matches both the spec and
the accuracy test by pm_rc6_residency.

v2:
- simplify logic checking for the CHV 320MHz special case (Rodrigo)

Testcase: igt/pm_rc6_residency
Signed-off-by: Imre Deak <imre.deak@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76877
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>