drm/i915/bxt: Modify BXT BLC according to VBT changes

Latest VBT mentions which set of registers will be used for BLC,
as controller number field. Making use of this field in BXT
BLC implementation. Also, the registers are used in case control
pin indicates display DDI. Adding a check for this.
According to Bspec, BLC_PWM_*_2 uses the display utility pin for output.
To use backlight 2, enable the utility pin with mode = PWM
   v2: Jani's review comments
   addressed
       - Add a prefix _ to BXT BLC registers definitions.
       - Add "bxt only" comment for u8 controller
       - Remove control_pin check for DDI controller
       - Check for valid controller values
       - Set pipe bits in UTIL_PIN_CTL
       - Enable/Disable UTIL_PIN_CTL in enable/disable_backlight()
       - If BLC 2 is used, read active_low_pwm from UTIL_PIN polarity
   Satheesh's review comment addressed
       - If UTIL PIN is already enabled, BIOS would have programmed it. No
       need to disable and enable again.
   v3: Jani's review comments
       - add UTIL_PIN_PIPE_MASK and UTIL_PIN_MODE_MASK
       - Disable UTIL_PIN if controller 1 is used
       - Mask out UTIL_PIN_PIPE_MASK and UTIL_PIN_MODE_MASK before enabling
       UTIL_PIN
       - check valid controller value in intel_bios.c
       - add backlight.util_pin_active_low
       - disable util pin before enabling
   v4: Change for BXT-PO branch:
   Stubbed unwanted definition which was existing before
   because of DC6 patch.
   UTIL_PIN_MODE_PWM     (0x1b << 24)

v2: Fixed Jani's review comment.

v3: Split the backight PWM frequency programming into separate patch,
    in cases BIOS doesn't initializes it.

v4: Starting afresh and not modifying existing state for backlight, as
    per Jani's recommendation.

v5: Fixed Jani's review comment wrt util pin enable

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Signed-off-by: Sunil Kamath <sunil.kamath@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>