drm/i915/cnl: Apply large line width optimization

This bit enables hardware that will change the approximation used for distances
calculations for AA wide lines so that they are rendered more accurately.

The default value for this bit leaves the legacy behavior. There is no good
reason to not enable the new approximation except if comparing to previous GEN
rendered images.

v2: Rebase
v3: Fix author.
    Rebased by Rodrigo who also added  a comment as suggested by Oscar.
    Since it is surrounded by Workarounds let's just add a comment to
    make clear it is not an Wa.

Cc: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Oscar Mateo <oscar.mateo@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170815231651.975-4-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/i915_reg.h

@@ -2491,6 +2491,7 @@ enum i915_power_well_id {
 # define _3D_CHICKEN2_WM_READ_PIPELINED			(1 << 14)
 #define _3D_CHICKEN3	_MMIO(0x2090)
 #define  _3D_CHICKEN_SF_DISABLE_OBJEND_CULL		(1 << 10)
+#define  _3D_CHICKEN3_AA_LINE_QUALITY_FIX_ENABLE	(1 << 5)
 #define  _3D_CHICKEN3_SF_DISABLE_FASTCLIP_CULL		(1 << 5)
 #define  _3D_CHICKEN_SDE_LIMIT_FIFO_POLY_DEPTH(x)	((x)<<1) /* gen8+ */
 #define  _3D_CHICKEN3_SF_DISABLE_PIPELINED_ATTR_FETCH	(1 << 1) /* gen6 */

drivers/gpu/drm/i915/intel_pm.c

@@ -8265,6 +8265,10 @@ static void gen8_set_l3sqc_credits(struct drm_i915_private *dev_priv,
 
 static void cannonlake_init_clock_gating(struct drm_i915_private *dev_priv)
 {
+	/* This is not an Wa. Enable for better image quality */
+	I915_WRITE(_3D_CHICKEN3,
+		   _MASKED_BIT_ENABLE(_3D_CHICKEN3_AA_LINE_QUALITY_FIX_ENABLE));
+
 	/* WaEnableChickenDCPR:cnl */
 	I915_WRITE(GEN8_CHICKEN_DCPR_1,
 		   I915_READ(GEN8_CHICKEN_DCPR_1) | MASK_WAKEMEM);