drm/i915/gvt: Correct the calculation of plane size

stride isn't in unit of pixel, it is bytes, so calculation of
plane size doesn't need to multiple bpp.

Fixes: e546e281 ("drm/i915/gvt: Dmabuf support for GVT-g")
Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>