drm/amdgpu: simplify the access to eeprom_control struct

simplify the code of accessing to eeprom_control struct

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>