drm/msm: dpu: Remove crtc_lock from setup_mixers

I think the intention here was to protect the enc->crtc access, but
that's insufficient to avoid enc->crtc changing. Fortunately we're
already holding the modeset lock when this is called (from
atomic_check), so remove the crtc_lock and add a modeset lock check.

While we're at it, use the encoder mask from crtc state instead of
legacy pointer.

Changes in v2:
- None

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

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