drm/msm/dsi: Queue HPD helper work in attach/detach callbacks

The msm/dsi host drivers calls drm_helper_hpd_irq_event in the
mipi_dsi_host attach/detatch callbacks.

mipi_dsi_attach()/mipi_dsi_detach() from a panel/bridge
driver could be called from a context where the drm_device's
mode_config.mutex is already held, resulting in a deadlock.
Queue it as work instead.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>