drm/msm: dpu: Add ->enabled to dpu_encoder_virt

Add a bool to dpu_encoder_virt to track whether the encoder is enabled
or not. Repurpose the enc_lock mutex to ensure that it is consistent
with the hw state.

Changes in v2:
- None

Cc: Jeykumar Sankaran <jsanka@codeaurora.org>
Reviewed-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>

Signed-off-by: Rob Clark <robdclark@gmail.com>