Revert "drm/i915: Add heuristic to determine better way to adjust brightness"

This reverts commit 560a758d39.

The DPCD backlight commits regress a Thinkpad X1 Carbon 4th Gen and a
BXT-P (in CI). Enabling dynamic backlight boots to a black screen, and
enabling DPCD backlight leads to a black screen after suspend/resume.

References: http://mid.mail-archive.com/20170706135349.6tu3lz7uehazlnnn@boom
References: http://mid.mail-archive.com/20170627132326.f2q3yn4bh5flji4q@boom
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101619
Reported-by: David Weinehall <david.weinehall@linux.intel.com>
Fixes: 560a758d39 ("drm/i915: Add heuristic to determine better way to adjust brightness")
Cc: Jenny TC <jenny.tc@intel.com>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Cc: Puthikorn Voravootivat <puthik@chromium.org>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: intel-gfx@lists.freedesktop.org
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f49a89a05f18e90871c2eeadcdcd783ac7961cdf.1500542254.git.jani.nikula@intel.com

drivers/gpu/drm/i915/i915_params.c

@@ -63,7 +63,7 @@ struct i915_params i915 __read_mostly = {
 	.huc_firmware_path = NULL,
 	.enable_dp_mst = true,
 	.inject_load_failure = 0,
-	.enable_dpcd_backlight = -1,
+	.enable_dpcd_backlight = false,
 	.enable_gvt = false,
 };
 
@@ -246,10 +246,9 @@ MODULE_PARM_DESC(enable_dp_mst,
 module_param_named_unsafe(inject_load_failure, i915.inject_load_failure, uint, 0400);
 MODULE_PARM_DESC(inject_load_failure,
 	"Force an error after a number of failure check points (0:disabled (default), N:force failure at the Nth failure check point)");
-module_param_named_unsafe(enable_dpcd_backlight, i915.enable_dpcd_backlight, int, 0600);
+module_param_named(enable_dpcd_backlight, i915.enable_dpcd_backlight, bool, 0600);
 MODULE_PARM_DESC(enable_dpcd_backlight,
-	"Enable support for DPCD backlight control "
-	"(-1:auto (default), 0:force disable, 1:force enabled if supported");
+	"Enable support for DPCD backlight control (default:false)");
 
 module_param_named(enable_gvt, i915.enable_gvt, bool, 0400);
 MODULE_PARM_DESC(enable_gvt,