[media] adv7604: improve EDID handling

- split edid_write_block()
- do not use edid->edid before the validity check
- Return -EINVAL if edid->pad is invalid
- Save both registers for SPA port A
- Set SPA location to default value if it is not found

Signed-off-by: Mats Randgaard <matrandg@cisco.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>