drm/tegra: Remove two unused function declarations

Commit 776dc384 ("drm/tegra: Move subdevice infrastructure to host1x")
removed the implementation but not the declaration.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230809030226.3412-1-yuehaibing@huawei.com