drm/omap: fix crash on module unload

When unloading omapdrm we get a NULL pointer deref in
omap_drm_irq_uninstall(). This is caused by:

967dd484 ("drm: remove
drm_vblank_no_hw_counter assignment from driver code")

We shut down all the crtcs at unload time before calling
omap_drm_irq_uninstall, so the code in omap_drm_irq_uninstall() is
unnecessary and can be removed.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>