[media] media: exynos4-is: fix deadlock on driver probe

Commit 0c426c47 ("[media] media: Always
keep a graph walk large enough around") changed
media_device_register_entity() function to take mdev->graph_mutex. This
causes deadlock in driver probe, which calls (indirectly) this function
with ->graph_mutex taken. This patch removes taking ->graph_mutex in
driver probe to avoid deadlock. Other drivers don't take ->graph_mutex
for entity registration, so this change should be safe.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Tested-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>