drm/msm/dsi: drop msm_dsi_device_connected() function

Since the commit 8f59ee9a ("drm/msm/dsi: Adjust probe order") the
DSI hosts are not bound through the component framework if the DSI
driver wasn't attached to the DSI device connected to this host.

Afterwards, if there is no bridge (including the panel bridge) created
for the DSI device then devm_drm_of_get_bridge() will return an error,
also making msm_dsi_manager_ext_bridge_init() and thus DSI modesetting
init fail.

This way there can be no 'unconnected' MSM DSI bridges. Remove the
msm_dsi_device_connected() function.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/561692/
Link: https://lore.kernel.org/r/20231009205727.2781802-3-dmitry.baryshkov@linaro.org