drm: msm: Quiet down plane errors in atomic_check

With the db845c running AOSP, I see the following error on every
frame on the home screen:
  [drm:dpu_plane_atomic_check:915] [dpu error]plane33 invalid src 2880x1620+0+470 line:2560

This is due to the error paths in atomic_check using
DPU_ERROR_PLANE(), and the drm_hwcomposer using atomic_check
to decide how to composite the frame (thus it expects to see
atomic_check to fail).

In order to avoid spamming the logs, this patch converts the
DPU_ERROR_PLANE() calls to DPU_DEBUG_PLANE() calls in
atomic_check.

Cc: Todd Kjos <tkjos@google.com>
Cc: Alistair Delva <adelva@google.com>
Cc: Amit Pundir <amit.pundir@linaro.org>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Cc: freedreno@lists.freedesktop.org
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>