drm/amd/powerplay: add check before i2c_add_adapter

smu_i2c_eeprom_init may be invoked twice or more
under sroiv mode, while we don't want to add check
if (!amdgpu_sriov_vf) before we invoke smu_i2c_eeprom_init/fini
each time, so we check if i2c adapter is already added
before we invoke i2c_add_adapter

Signed-off-by: Wenhui Sheng <Wenhui.Sheng@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>