drm/msm: dpu: Stop using encoder->crtc pointer

It's for legacy drivers, for atomic drivers crtc->state->encoder_mask
should be used to map encoder to crtc.

Changes in v2:
- None

Reviewed-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
[seanpaul resolved conflict with async param of dpu_encoder_kickoff]

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