[media] mt9m111: fix Oops - initialise context before dereferencing

A recent commit "[media] soc-camera: Push probe-time power management to
drivers" causes an Oops during mt9m111 driver probing because its .ctx
private data field is now dereferenced before it is initialised. Fix this
by initialising the field earlier.

Reported-by: Javier Martin <javier.martin@vista-silicon.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Tested-by: Javier Martin <javier.martin@vista-silicon.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>