drm/i915: Add Backlight Control using DPCD for eDP connectors (v9)

This patch adds support for eDP backlight control using DPCD registers to
backlight hooks in intel_panel.

It checks for backlight control over AUX channel capability and sets up
function pointers to get and set the backlight brightness level if
supported.

v2: Moved backlight functions from intel_dp.c into a new file
intel_dp_aux_backlight.c. Also moved reading of eDP display control
registers to intel_dp_get_dpcd

v3: Correct some formatting mistakes

v4: Updated to use AUX backlight control if PWM control is not possible
	(Jani)
v5: Moved call to initialize backlight registers to dp_aux_setup_backlight
v6: Check DP_EDP_BACKLIGHT_PIN_ENABLE_CAP is disabled before setting up AUX
backlight control. To fix BLM_PWM_ENABLE igt test warnings on bdw_ultra
v7: Add enable_dpcd_backlight module parameter.
v8: Rebase onto latest drm-intel-nightly branch
v9: Remove changes to intel_dp_dpcd_read_wake
    Split addition edp_dpcd variable into a separate patch

Cc: Bob Paauwe <bob.j.paauwe@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Yetunde Adebisi <yetundex.adebisi@intel.com>
[Jani: whitepace changes to appease checkpatch]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459865452-9138-4-git-send-email-yetundex.adebisi@intel.com