drm/msm: dpu: Consolidate atomic_check functions()

dpu_plane_atomic_check() is a very thin wrapper around
dpu_plane_sspp_atomic_check(). All it does is a NULL-check of state->fb,
which is already done by drm_atomic_helper_check_plane_state(). Further,
the helper sets state->visible = false when this is true. So remove
dpu_plane_atomic_check() and just use dpu_plane_sspp_atomic_check()
directly.

Changes in v2:
- Fix spelling mistake in Subject (Jeykumar)

Reviewed-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>