drm/msm/dpu: drop VBIF index from the struct dpu_hw_vbif

drm/msm/dpu: drop VBIF index from the struct dpu_hw_vbif

Since we don't support and don't use VBIF_NRT, VBIF_RT is the only
possible VBIF type. To simplify the driver, drop vbif_idx from the VBIF
instance structure. As the last users of VBIF_RT and enum dpu_vbif are
gone, drop them too.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/707779/
Link: https://lore.kernel.org/r/20260227-drop-vbif-nrt-v1-7-2b97d0438182@oss.qualcomm.com