drm/msm/dsi: Don't call hpd helper unconditionally

Currently, msm dsi_host's attach/detach ops call drm_helper_hpd_irq_event
unconditionally. Calling it makes sense if we have a drm_panel
connected (in this case, we created the connector), but not when we have
a bridge chip connected. For bridges, the driver (or someone else in the
encoder chain) should call the hpd handler.

Call it in the attach/detach ops only if we're connected to a panel.

Signed-off-by: Archit Taneja <architt@codeaurora.org>