drm/msm: dpu: Remove dpu_plane_enabled()

plane->state->visible encompasses all of these checks and more, so we
can just check visible.

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>