media: platform: mtk-mdp3: work around unused-variable warning

When CONFIG_OF is disabled, the 'data' variable is not used at all
because of_match_node() turns into a dummy macro:

drivers/media/platform/mediatek/mdp3/mtk-mdp3-comp.c: In function 'mdp_comp_sub_create':
drivers/media/platform/mediatek/mdp3/mtk-mdp3-comp.c:1038:36: error: unused variable 'data' [-Werror=unused-variable]
 1038 |  const struct mtk_mdp_driver_data *data = mdp->mdp_data;
      |                                    ^~~~

Remove the variable again by moving the pointer dereference into the
of_match_node call.

Fixes: b385b991 ("media: platform: mtk-mdp3: chip config split about subcomponents")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>