drm/i915: Fix an error code i915_gem_object_copy_blt()

This code should use "vma[1]" instead of "vma".  The "vma" variable is a
valid pointer.

Fixes: 6b050304 ("drm/i915: Convert i915_gem_object/client_blt.c to use ww locking as well, v2.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200911075243.GG12635@kadam