drm/msm/mdp5: Make the intf connection in config module

Up until now, we assume that eDP is tight to intf_0 and HDMI to
intf_3. This information shall actually come from the mdp5_cfg
module since it can change from one chip to another.

v2: rename macro to mdp5_cfg_intf_is_virtual() [pointed by Archit]
v3: add sanity check before writing in INTF_TIMING_ENGINE_EN registers

Signed-off-by: Stephane Viau <sviau@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>