drm/msm/dpu: Use dedicated WB number definition

drm/msm/dpu: Use dedicated WB number definition

Currently MAX_CHANNELS_PER_ENC is defined as 2, because 2 channels are
supported at most in one encoder. The case of 4 channels per encoder is
to be added. To avoid breaking current WB usage case, use dedicated WB
definition before 4 WB usage case is supported in future.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/675410/
Link: https://lore.kernel.org/r/20250918-v6-16-rc2-quad-pipe-upstream-4-v16-6-ff6232e3472f@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>