drm/amdgpu: Fix width of I2C address

The I2C address is kept as a 16-bit quantity in
the kernel. The I2C_TAR::I2C_TAR field is 10-bit
wide.

Fix the width of the I2C address for Vega20 from 8
bits to 16 bits to accommodate the full spectrum
of I2C address space.

Cc: Jean Delvare <jdelvare@suse.de>
Cc: Alexander Deucher <Alexander.Deucher@amd.com>
Cc: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Cc: Lijo Lazar <Lijo.Lazar@amd.com>
Cc: Stanley Yang <Stanley.Yang@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Alexander Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>