drm/msm: Attach assigned encoder to eDP and DSI connectors

drm_mode_connector_attach_encoder() function call is missing
during eDP and DSI connector initialization. As a result,
no encoder is returned by DRM_IOCTL_MODE_GETCONNECTOR system
call. This change is to fix this issue.

Signed-off-by: Hai Li <hali@codeaurora.org>