drm/msm/dpu: check both DPU and MDSS devices for the IOMMU

drm/msm/dpu: check both DPU and MDSS devices for the IOMMU

Follow the lead of MDP5 driver and check both DPU and MDSS devices for
the IOMMU specifiers.

Historically DPU devices had IOMMU specified in the MDSS device tree
node, but as some of MDP5 devices are being converted to the supported
by the DPU driver, the driver should adapt and check both devices.

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/489696/
Link: https://lore.kernel.org/r/20220616081106.350262-2-dmitry.baryshkov@linaro.org
Signed-off-by: Rob Clark <robdclark@chromium.org>