drm/i915/cnp: Backlight support for CNP.

Split out BXT and CNP's setup_backlight(),enable_backlight(),
disable_backlight() and hz_to_pwm() into
two separate functions instead of reusing BXT function.

Reuse set_backlight() and get_backlight() since they have
no reference to the utility pin.

v2: Reuse BXT functions with controller 0 instead of
    redefining it. (Jani).
    Use dev_priv->rawclk_freq instead of getting the value
    from SFUSE_STRAP.
v3: Avoid setup backligh controller along with hooks and
    fully reuse hooks setup as suggested by Jani.
v4: Clean up commit message.
v5: Implement per PCH instead per platform.

v6: Introduce a new function for CNP.(Jani and Ville)

v7: Squash the all CNP Backlight support patches into a
single patch. (Jani)

v8: Correct indentation, remove unneeded blank lines and
correct mail address (Jani).

v9: Remove unused enum pipe. (by CI)

v10: Remove comment mentioning SFUSE_STRAP in a part of
     the code that we don't use it. (Jani)
     Make controller = 0 since current CNP has only one
     controller and put a comment mentioning why we
     reuse the BXT definitions and are keeping the
     controller = 0. (DK)
v11: Remove spurious line. (DK)

Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Suggested-by: Jani Nikula <jani.nikula@intel.com>
Suggested-by: Ville Syrjala <ville.syrjala@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1496434004-29812-4-git-send-email-rodrigo.vivi@intel.com