drm/i915: fix use of uninitialized pointer vaddr

The assignment of err is using the incorrect pointer vaddr that has
not been initialized. Fix this by using the correct pointer obj instead.

Addresses-Coverity: ("Uninitialized pointer read")
Fixes: 6501aa4e ("drm/i915: add in-kernel blitter client")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190531103201.10124-1-colin.king@canonical.com