media: mtk-mdp: address a clang warning

The typecasts at the dvb-core generate clang warnings when W=1
is enabled.

Such warning is harmless, but it causes the build to break with
CONFIG_WERROR and W=1 with clang, so do the cast on a way that
it won't produce warnings anymore.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>