media: omap4iss: Acquire graph mutex for graph traversal

Acquire the graph_mutex for traversing the media graph. This seems to have
been missing all the time.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>