drm/hisilicon: Remove unused delarations

hibmc_mm_init() has been removed since commit 28645ae0
("drm/hisilicon/hibmc: Remove hibmc_ttm.c"), but remain the declaration
untouched in the header files. So, let's remove this unused declaration.

Signed-off-by: Zhang Zekun <zhangzekun11@huawei.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240812123543.64300-1-zhangzekun11@huawei.com