drm/i915: Fix VBT backlight Hz to PWM conversion for PNV

Convert the MHz number coming from intel_rawclk() into Hz in
i9xx_hz_to_pwm() on PNV. Otherwise we'll get something totally
bogus as a result.

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/1448893432-6978-2-git-send-email-ville.syrjala@linux.intel.com


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