Please let us know more clear on this "The register address of the ADV7280A-M is 8 bits, but the address on the I2C bus is 7 bits". Its bit confusing.
see Table 99:User Sub Map Register Map Details.(User-Guide)
I2c driver, serial data (SDATA) and serial clock (SCLK), carry information between the ADV728x(slave) and the system I2C master controller. So the ADV728x main Map has possible slave addresses for read and write operation with corresponding master chip, In this case only ,the i2c driver will be 7-bit +R/W bit (Master-Slave Communication) between two chips.
But user Sub Map register(SUB_USR_EN) is different,(i.e) that can be accessed via the main i2c ports of ADV728x. Each map has its own I2C address with 8 bit addressing.
To access the User Sub Map, set the SUB_USR_EN bits(0x0E[6:5]) as 00. User Sub Map has the same I2C slave address as the Main Map(i.e Main map address 0x42)
For Example - 42 0E 00; //To enter User Sub Map
Note: The user sub map contains register that program the analog frond end and digital core of the ADV728x.
To write to 0x8E, what kind of data should be loaded on the sub address (7bit) on I2C and R / Wbit?
Free-Run Line Length1(0x8F) is a read/write register.The default value of 0x8F is 0x00,that is 27MHz is selected on LLC pin. When free run line length1 configured as 50(i.e.42 8F 50) - 13.5 MHz LLC selected out on LLC pin. To do this you need to set 0x8F[6:4] as 101.Please refer below