GFX-Display: change GI PWM frequnce to 1KHZ.

GFX-Display: change GI PWM frequnce to 1KHZ.

BZ: 26690

the orginal is 20KHZ, which cause backlight to
filker when display not at 100%.

Num of PWM / 1frame = (BCFRQSEL + 1) * 2 ^ BL_DIMM_STEP

here set BCFRQSEL = 16 ==> 00010000
BL_DIMM_STEP = 1 ==> 000

Change-Id: Ia9329f8bbbe3b72d887366a9af89fbacd7feb523
Signed-off-by: Ke Ai <ke.ai@intel.com>
Reviewed-on: http://android.intel.com:8080/38042
Reviewed-by: Tong, BoX <box.tong@intel.com>
Tested-by: Tong, BoX <box.tong@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>