drm/amd/display: initialize i2c speed if not initialized in dcnxxx__resource.c

Some dcnxxx__resource.c do not initialize the i2c speed; this patch adds
the required initialization at dc_construct().

Signed-off-by: Brendan Steve Leder <brendanSteve.Leder@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>