drm/msm/dpu: drop redundant num_planes assignment in _dpu_format_populate_plane_sizes*()

drm/msm/dpu: drop redundant num_planes assignment in _dpu_format_populate_plane_sizes*()

Drop redundant layout->num_planes assignments, using the value assigned
from the formats table. RGB UBWC formats need special handling: they use
two planes (per the format table), but the uAPI defines plane[1] as
empty.

Reviewed-by: Jessica Zhang <jessica.zhang@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/688180/
Link: https://lore.kernel.org/r/20251114-dpu-formats-v3-10-cae312379d49@oss.qualcomm.com
Tested-by: Luca Weiss <luca.weiss@fairphone.com> # qcm6490-fairphone-fp5
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>