drm/i915: clean up Hz to PWM for i965

Unify function structure as any other *_hz_to_pwm() functions are
structured.

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1481616169-16210-4-git-send-email-mika.kahola@intel.com