drm/msm: dpu: Remove vblank_callback from encoder

The indirection of registering a callback and opaque pointer isn't reall
useful when there's only one callsite. So instead of having the
vblank_cb registration, just give encoder a crtc and let it directly
call the vblank handler.

In a later patch, we'll make use of this further.

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>