drm/nouveau/acr: Abort loading ACR if no firmware was found

This fixes a NULL pointer access inside nvkm_acr_oneinit in case necessary
firmware files couldn't be loaded.

Closes: drm/nouveau#212


Fixes: 4b569ded ("drm/nouveau/acr/ga102: initial support")
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230522201838.1496622-1-kherbst@redhat.com