drm/msm/dsi: Correct io_start for MSM8994 (20nm PHY)

The previous registers were *almost* correct, but instead of
PHYs, they were pointing at DSI PLLs, resulting in the PHY id
autodetection failing miserably.

Fixes: dcefc117 ("drm/msm/dsi: Add support for msm8x94")
Signed-off-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>