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

This fixes a warning found when building with gcc7:

drivers/media/platform/qcom/venus/hfi_venus.c:998
venus_isr_thread() warn: variable dereferenced before check
'hdev' (see line 994)

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>