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