media: mtk-mdp: fix kernel-doc warnings

mtk_mdp_comp.h: MTK_MDP_COMP_TYPE_MAX was undocumented, but it
is also unused, so just remove it.

mtk_mdp_core.h: add missing '*' inside comment blocks, add a
missing : after '@ctrls' and add a missing struct keyword.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: Eizan Miyamoto <eizan@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>