[media] media-entity: only call dev_dbg_obj if mdev is not NULL

Fix kernel Oops NULL pointer deference
Call dev_dbg_obj only after checking if gobj->mdev is not NULL

Signed-off-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>