drm/amdgpu/pm: rework i2c xfers on sienna cichlid (v4)

Make it generic so we can support more than just EEPROMs.

v2: fix restart handling between transactions.
v3: handle 7 to 8 bit addr conversion
v4: Fix &req --> req. (Luben T)

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