media: venus: helpers: drop setting of timestamp invalid flag

The zero timestamp is really valid so fix that mistake by
dropping the code which checks for zero timestamp.

Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Tested-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>