drm/msm: Set encoder's mode of operation using a kms func

The mdp5 kms driver currently sets up multiple encoders per interface
(INTF), one for each kind of mode of operation it supports.
We create 2 drm_encoders for DSI, one for Video Mode and the other
for Command Mode operation. The reason behind this approach could have
been that we aren't aware of the DSI device's mode of operation when
we create the encoders.

This makes things a bit complicated, since these encoders have to
be further attached to the same DSI bridge. The easier way out is
to create a single encoder, and make the DSI driver set its mode
of operation when we know what the DSI device's mode flags are.

Start with providing a way to set the mdp5_intf_mode using a kms
func that sets the encoder's mode of operation. When constructing
a DSI encoder, we set the mode of operation to Video Mode as
default. When the DSI device is attached to the host, we probe the
DSI mode flags and set the corresponding mode of operation.

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