mtk-vcodec: Use designated initializers

The randstruct plugin requires designated initializers for structures
that are entirely function pointers.

Cc: Wu-Cheng Li <wuchengli@chromium.org>
Cc: Tiffany Lin <tiffany.lin@mediatek.com>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Kees Cook <keescook@chromium.org>