OMAPDSS: fix potential NULL pointer ref in OCP_ERR handling path

The dispc's error handler tries to disable all outputs when OCP_ERR
happens. However, the code doesn't check if there actually is a display
on each particular output, nor if there's a driver for the display. This
may lead to NULL pointer reference.

Signed-off-by: Rob Clark <rob@ti.com>
[tomi.valkeinen@ti.com: added patch description]
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>