drm/msm/dsi: simplify connector creation

Instead of having two functions, msm_dsi_manager_bridge_init()
and msm_dsi_manager_ext_bridge_init(), merge them into
msm_dsi_manager_connector_init(), moving drm_bridge_attach() to be
called from the bridge's attach callback (as most other bridges do).

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/582212/
Link: https://lore.kernel.org/r/20240309-fd-dsi-cleanup-bridges-v1-3-962ebdba82ed@linaro.org