drm/amd/display: Correct I2C register offset

[Why]
I2C register name starts with 1, unlike other registers that start with
0. This creates a problem with the new register macro refactoring when
I2C HW objects are created in an array.

[How]
Correct I2C register offset by making a new macro to account for array
offset.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Pavle Kotarac <Pavle.Kotarac@amd.com>
Signed-off-by: Chris Park <chris.park@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>