drm/msm/dpu: dont use encoder->crtc in atomic path

encoder->crtc is not really meaningful for atomic path. Use
crtc->encoder_mask to identify the crtc attached with
an encoder.

Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1550107156-17625-5-git-send-email-jsanka@codeaurora.org


Signed-off-by: Rob Clark <robdclark@chromium.org>