drm/omap: fix: change dev_unload order

The current dev_unload order uninits the irqs too early.

In the current sequence, it's possible that a crtc queues work(apply_worker)
to display a buffer, which registers to omap_crtc_apply_irq to notfiy the
completion of the configuration we applied.

Calling drm_vblank_cleanup and omap_drm_irq_uninstall here causes the crtc's
apply handler to never get called, which results in an incorrect state of the
apply_irq.registered parameter.

This condition occurs where there is no mode set via omapdrm, and dev_lastclose
tries to set a default fb mode via drm_fb_helper_restore_fbdev_mode. The apply
work scheduled by restore_fbdev_mode is very close in time to the disabling of
the irq handler, and hence leads to a race condition. We move the irq cleanup
at the end of the unload sequence to prevent this.

Also, the call to flush_workqueue is removed since it's called internally by
destroy_workqueue.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>