gpu: ipu-v3: fix a possible NULL dereference

of_match_device could return NULL, and so cause a NULL pointer
dereference later.

For fixing this problem, we use of_device_get_match_data(), this will
simplify the code a little by using a standard function for
getting the match data.

Testing the return value of of_device_get_match_data is also necessary
for avoiding a second NULL deref later on devtype.

Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>