drm/msm/mdp5: Create single encoder per interface (INTF)

For the DSI interfaces, the mdp5_kms core creates 2 encoders for video
and command modes.

Create only a single encoder per interface. When creating the encoder, set
the interface type to MDP5_INTF_MODE_NONE. It's the bridge (DSI/HDMI/eDP)
driver's responsibility to set a different interface type. It can use the
the kms func op set_encoder_mode to change the mode of operation, which
in turn would configure the interface type for the INTF.

In mdp5_cmd_encoder.c, we remove the redundant code, and make the commmand
mode funcs as helpers that are used in mdp5_encoder.c

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>