drm/i915/gvt: Fix cmd length check for MI_ATOMIC

Correct valid command length check for MI_ATOMIC, need to check inline
data available field instead of operand data length for whole command.

Fixes: 00a33be4

 ("drm/i915/gvt: Add valid length check for MI variable commands")
Reported-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Gao Fred <fred.gao@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>