drm/amdgpu: Remember to wait 10ms for write buffer flush v2

EEPROM spec requests this.

v2: Only to be done for write data transactions.

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