drm/i915: Fix vbt PWM max setup for CTG

CTG uses hrawclk for backlight, so calculate the max based on that
instead of cdclk.

Fixes: aa17cdb4 ("drm/i915: initialize backlight max from VBT")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448893432-6978-3-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>