drm/loongson: use GEM references instead of TTMs

Instead of a TTM reference grab a GEM reference whenever necessary.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sui Jingfeng <suijingfeng@loongson.cn>
Link: https://patchwork.freedesktop.org/patch/msgid/20240723121750.2086-3-christian.koenig@amd.com