drm/i915/gen9: fix the WM memory bandwidth WA for Y tiling cases

The previous spec version said "double Ytile planes minimum lines",
and I interpreted this as referring to what the spec calls "Y tile
minimum", but in fact it was referring to what the spec calls "Minimum
Scanlines for Y tile". I noticed that Mahesh Kumar had a different
interpretation, so I sent and email to the spec authors and got
clarification on the correct meaning. Also, BSpec was updated and
should be clear now.

Fixes: ee3d532fcb64 ("drm/i915/gen9: unconditionally apply the memory bandwidth WA")
Cc: stable@vger.kernel.org
Cc: Mahesh Kumar <mahesh1.kumar@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1478636531-6081-1-git-send-email-paulo.r.zanoni@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -3626,6 +3626,9 @@ static int skl_compute_plane_wm(const struct drm_i915_private *dev_priv,
 		y_min_scanlines = 4;
 	}
 
+	if (apply_memory_bw_wa)
+		y_min_scanlines *= 2;
+
 	plane_bytes_per_line = width * cpp;
 	if (fb->modifier[0] == I915_FORMAT_MOD_Y_TILED ||
 	    fb->modifier[0] == I915_FORMAT_MOD_Yf_TILED) {
@@ -3646,8 +3649,6 @@ static int skl_compute_plane_wm(const struct drm_i915_private *dev_priv,
 				 plane_blocks_per_line);
 
 	y_tile_minimum = plane_blocks_per_line * y_min_scanlines;
-	if (apply_memory_bw_wa)
-		y_tile_minimum *= 2;
 
 	if (fb->modifier[0] == I915_FORMAT_MOD_Y_TILED ||
 	    fb->modifier[0] == I915_FORMAT_MOD_Yf_TILED) {