[media] media: venus: helpers: fix variable dereferenced before check

This fixes a warning found when building the driver with gcc7:

drivers/media/platform/qcom/venus/helpers.c:157
load_per_instance() warn: variable dereferenced before check
'inst' (see line 153)

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>