drm/msm/mdp5: Clean up interface assignment

mdp5_interface struct contains data corresponding to a INTF
instance in MDP5 hardware. This sturct is memcpy'd to the
mdp5_encoder struct, and then later to the mdp5_ctl struct.

Instead of copying around interface data, create mdp5_interface
instances in mdp5_init, like how it's done currently done for
pipes and layer mixers. Pass around the interface pointers to
mdp5_encoder and mdp5_ctl. This simplifies the code, and allows
us to decouple encoders from INTFs in the future if needed.

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