[media] Corrected Oops on omap_vout when no manager is connected

If no manager is connected to the vout device, the omapvid_init() function
fails. No error condition is checked, and the device is started. Later on,
when irq is serviced, a NULL pointer dereference occurs.
Also, the isr routine must be registered only if no error occurs, otherwise
the isr triggers without the proper setup, and the kernel oops again.
To prevent this, the error condition is checked, and the streamon function
exits with error. Also the isr registration call is moved after the setup
procedure is completed.

Reviewed-by: Prabhakar Lad <prabhakar.lad@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>