dt-bindings: display: msm/dsi: Remove unused properties

"qcom,dsi-host-index" and "qcom,dsi-phy-index" DT props aren't
acceptable and have never been used in any DT files. Remove them.

Cc: Rob Herring <robh@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>