drm/msm/dpu: drop VBIF_NRT handling

drm/msm/dpu: drop VBIF_NRT handling

The second VBIF instance, VBIF_NRT, is only used for the separate inline
rotator block. It is unsupported by the DPU driver and will require a
separate driver (or separate instance of the DPU device).

The only possible user of VBIF_NRT is writeback on MSM8996, however
writeback on that platform is currently unsupported and it's not worth
keeping extra complexity for the sake of that single legacy platform.

None of the hardware catalogs entries actually declare VBIF_NRT, so it
is left in its default state.

Stop pretending that DPU driver cares about VBIF_NRT and drop it.

Reported-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
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/707773/
Link: https://lore.kernel.org/r/20260227-drop-vbif-nrt-v1-1-2b97d0438182@oss.qualcomm.com