drm/amd/powerplay: Make two functions static

Fix sparse warnings:

drivers/gpu/drm/amd/amdgpu/../powerplay/arcturus_ppt.c:2050:5:
 warning: symbol 'arcturus_i2c_eeprom_control_init' was not declared. Should it be static?
drivers/gpu/drm/amd/amdgpu/../powerplay/arcturus_ppt.c:2068:6:
 warning: symbol 'arcturus_i2c_eeprom_control_fini' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>