drm/amdgpu: add new helper for handling EEPROM i2c transfers

Encapsulates the i2c protocol handling so other parts of the
driver can just tell it the offset and size of data to write.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>