media: mtk: fix mtk-smi dependency

The mtk-smi driver can now be built as a loadable module, but
this leads to a build time regression when the drivers that
depend on it are built-in:

aarch64-linux-ld: drivers/media/platform/mtk-mdp/mtk_mdp_comp.o: in function `mtk_mdp_comp_clock_on':
mtk_mdp_comp.c:(.text.mtk_mdp_comp_clock_on+0x54): undefined reference to `mtk_smi_larb_get'
aarch64-linux-ld: drivers/media/platform/mtk-mdp/mtk_mdp_comp.o: in function `mtk_mdp_comp_clock_off':
mtk_mdp_comp.c:(.text.mtk_mdp_comp_clock_off+0x12c): undefined reference to `mtk_smi_larb_put'

Add a dependency on the interface, but keep allowing
compile-testing without that driver, as it was originally
intended.

Fixes: 50fc8d92 ("memory: mtk-smi: Allow building as module")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>