drm/msm/dpu: avoid querying for hw intf before assignment

Resource manager assigns hw_intf blocks for the encoder only on
modeset. If queried for hw_intf objects during init, it will be
NULL. Since hw_intf objects are needed only after encoder enable,
defer the query to encoder enable which will be triggered after
modeset.

changes in v4:
	- Add details on commit text on why the change is needed (Sean)
changes in v5:
	- Reword commit text on the usage of hw_intf objects (Sean)

Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>