drm/msm/sde: Prevent modeset resource management on encoders in off state

Currently pre and post modeset resource management can be performed on
virtual encoders regardless of them having a physical encoder tied to
them. This operation is invalid, and can result in cascading and fatal
errors occurring during an atomic_commit.

Update the SDE encoder resource control to quietly return from pre and
post modeset management when the encoder is in the OFF state. This
prevents improper attempts to access the physical encoder, and allows
atomic commit execution to continue as intended.

Change-Id: I4292742dfbde2a611474d3921a8da7d95eff0f12
Signed-off-by: Christopher Braga <cbraga@codeaurora.org>