drm/amdgpu: Correctly disable the I2C IP block

On long transfers to the EEPROM device,
i.e. write, it is observed that the driver aborts
the transfer.

The reason for this is that the driver isn't
patient enough--the IC_STATUS register's contents
is 0x27, which is MST_ACTIVITY | TFE | TFNF |
ACTIVITY. That is, while the transmission FIFO is
empty, we, the I2C master device, are still
driving the bus.

Implement the correct procedure to disable
the block, as described in the DesignWare I2C
Databook, section 3.8.3 Disabling DW_apb_i2c on
page 56. Now there are no premature aborts on long
data transfers.

Cc: Alexander Deucher <Alexander.Deucher@amd.com>
Cc: Andrey Grodzovsky <Andrey.Grodzovsky@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>