Add phantom slice support on IVB+

Add phantom slice support on IVB+

HW requires driver to add a phantom slice when FirstMbX and FirstMbY are
not 0, in order to avc decoding error concealment. Otherwise, GPU may hang.
This patch is a workround for bug: https://bugs.freedesktop.org/show_bug.cgi?id=81447

v1->v2: Follow haihao's suggestion to use common funcion of gen6 phantom slice.
v2->v3: Remove an extraneous newline.

Signed-off-by: Zhong Li <zhong.li@intel.com>
Reviewed-by: Xiang Haihao <haihao.xiang@intel.com>
             Gwenole Beauchesne <gwenole.beauchesne@intel.com>
(cherry picked from commit 9672c5ab17c32f25ce1bbdb883abda689440b116)