drm/i915: Kill WA 0826

According to BSpec this is not needed anymore:

"This workaround is no longer needed since NV12
support is dropped for the affected projects.
"

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
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/20181031162845.12419-2-rodrigo.vivi@intel.com