drm/amdgpu: Fix Vega20 I2C to be agnostic (v2)

Teach Vega20 I2C to be agnostic. Allow addressing
different devices while the master holds the bus.
Set STOP as per the controller's specification.

v2: Qualify generating ReSTART before the 1st byte
    of the message, when set by the caller, as
    those functions are separated, as caught by
    Andrey G.

Cc: Jean Delvare <jdelvare@suse.de>
Cc: Alexander Deucher <Alexander.Deucher@amd.com>
Cc: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Cc: Lijo Lazar <Lijo.Lazar@amd.com>
Cc: Stanley Yang <Stanley.Yang@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Alexander Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>