drm/amd/display: init hw i2c speed

[Why]
Driver didn't init hw i2c speed cause hdcp hw cannot
send command, because the default value of speed register
is 0x2.

[How]
Restore the default speed when release i2c engine

Signed-off-by: Lewis Huang <Lewis.Huang@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>