drm/msm/dp: Add support for determining the eDP/DP mode from DT

drm/msm/dp: Add support for determining the eDP/DP mode from DT

Instead of relying on different compatibles for eDP and DP, lookup
the panel node in devicetree to figure out the connector type and
then pass on that information to the PHY. External DP doesn't have
a panel described in DT, therefore, assume it's eDP if panel node
is present.

Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/584534/
Link: https://lore.kernel.org/r/20240324-x1e80100-display-refactor-connector-v4-1-e0ebaea66a78@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>