[media] media: venus: vdec: fix compile error in vdec_close

This fixes the following compile error ocured when building
with gcc7:

drivers/media/platform/qcom/venus/vdec.c:1022
vdec_close() error: dereferencing freed memory 'inst'

by moving kfree as a last call.

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>