drm/nouveau/gr/gf100: Fix missing unlock in gf100_gr_chan_new()

When the call to gf100_grctx_generate() fails, unlock gr->fecs.mutex
before returning the error.

Fixes smatch warning:

drivers/gpu/drm/nouveau/nvkm/engine/gr/gf100.c:480 gf100_gr_chan_new() warn: inconsistent returns '&gr->fecs.mutex'.

Fixes: ca081fff ("drm/nouveau/gr/gf100-: generate golden context during first object alloc")
Signed-off-by: Li Huafei <lihuafei1@huawei.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241026173844.2392679-1-lihuafei1@huawei.com