drm/i915/gvt: Roundup fb->height into tile's height at calucation fb->size

When fb is tiled and fb->height isn't the multiple of tile's height,
the format fb->size = fb->stride * fb->height, will get a smaller size
than the actual size. As the memory height of tiled fb should be multiple
of tile's height.

Fixes: 7f1a93b1 ("drm/i915/gvt: Correct the calculation of plane size")
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>