[media] VPU: mediatek: fix null pointer dereference on pdev

pdev is being null checked, however, prior to that it is being
dereferenced by platform_get_drvdata.  Move the assignments of
vpu and run to after the pdev null check to avoid a potential
null pointer dereference.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Tiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>