drm/i915: Re-enable FBC WM if the watermark is good on gen6+

If the calculated FBC watermark is no good, we simply disable FBC
watermarks. But we fail to re-enable them later if the calculated
watermark becomes good again. Fix that, but remember to leave FBC
watermarks disabled on ILK since that's required by some workarounds.

v2: Fix checkpatch complaint

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -1631,6 +1631,10 @@ static bool ironlake_check_srwm(struct drm_device *dev, int level,
 		I915_WRITE(DISP_ARB_CTL,
 			   I915_READ(DISP_ARB_CTL) | DISP_FBC_WM_DIS);
 		return false;
+	} else if (INTEL_INFO(dev)->gen >= 6) {
+		/* enable FBC WM (except on ILK, where it must remain off) */
+		I915_WRITE(DISP_ARB_CTL,
+			   I915_READ(DISP_ARB_CTL) & ~DISP_FBC_WM_DIS);
 	}
 
 	if (display_wm > display->max_wm) {