drm/i915: Set PWM divider to match desired frequency in vbt

Read desired PWM frequency from panel vbt and calculate the
value for divider in DPCD address 0x724 and 0x728 to have
as many bits as possible for PWM duty cyle for granularity of
brightness adjustment while the frequency divisor is still
within 25% of the desired value.

Signed-off-by: Puthikorn Voravootivat <puthik@chromium.org>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170622190339.142671-2-puthik@chromium.org