drm/i915: Define WA 0870 and kill dead code.

Let's introduce the WA number that is the
cause of having NV12 disabled on both SLK and BXT.

According to Spec:

WA 0870: "Display flickers with NV12 video playback in
Y tiling mode.
WA: Use YUV422 surface format instead of NV12."

v2: remove the useless dead code and consequently
    avoiding device info flag. (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181031162845.12419-3-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/intel_sprite.c

@@ -1868,6 +1868,7 @@ static bool skl_plane_has_planar(struct drm_i915_private *dev_priv,
 	if (INTEL_GEN(dev_priv) >= 11)
 		return plane_id <= PLANE_SPRITE3;
 
+	/* Display WA #0870: skl, bxt */
 	if (IS_SKYLAKE(dev_priv) || IS_BROXTON(dev_priv))
 		return false;