nouveau/gsp/r535: Fix a NULL vs error pointer bug

The r535_gsp_cmdq_get() function returns error pointers but this code
checks for NULL.  Also we need to propagate the error pointer back to
the callers in r535_gsp_rpc_get().  Returning NULL will lead to a NULL
pointer dereference.

Fixes: 176fdcbd

 ("drm/nouveau/gsp/r535: add support for booting GSP-RM")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Danilo Krummrich <dakr@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f71996d9-d1cb-45ea-a4b2-2dfc21312d8c@kili.mountain