drm/i915/gt: Potential error pointer dereference in pinned_context()

If the intel_engine_create_pinned_context() function returns an error
pointer, then dereferencing "ce" will Oops.  Use "vm" instead of
"ce->vm".

Fixes: cf586021 ("drm/i915/gt: Pipelined page migration")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210813113600.GC30697@kili