nouveau: handle EBUSY and EAGAIN for GSP aux errors.

The upper layer transfer functions expect EBUSY as a return
for when retries should be done.

Fix the AUX error translation, but also check for both errors
in a few places.

Fixes: eb284f4b ("drm/nouveau/dp: Honor GSP link training retry timeouts")
Cc: stable@vger.kernel.org
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241111034126.2028401-1-airlied@gmail.com