drm/msm/dpu: drop vbif_idx from WB configuration

drm/msm/dpu: drop vbif_idx from WB configuration

All MDP / DPU implementations except for MSM8996 use VBIF_RT (or the
only VBIF) for WB2. Writeback on MSM8996 is not supported (nor planned
to be supported). In order to simplify the driver, drop the field form
the struct dpu_wb_cfg.

Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/707778/
Link: https://lore.kernel.org/r/20260227-drop-vbif-nrt-v1-5-2b97d0438182@oss.qualcomm.com
[DB: also handled Eliza platform]
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>