utests: Fix a bug in movforphi test case.

utests: Fix a bug in movforphi test case.

This test case is to trigger a old MovForPHI bug, and it
just use read/write_image. But it has a bug in itself.
As in the kernel, the write image will only write the
first lane data not all the 16 lanes. As the previous
patch fix the write_image bug, thus now the write_image
work correctly and thus it only touch the first data element
thus it trigger the bug in this test case. Now fix it.

Signed-off-by: Zhigang Gong <zhigang.gong@linux.intel.com>