media: venus: fix multiple encoder crash

Currently we are considering the instances which are available
in core->inst list for load calculation in min_loaded_core()
function, but this is incorrect because by the time we call
decide_core() for second instance, the third instance not
filled yet codec_freq_data pointer.

Solve this by considering the instances whose session has started.

Cc: stable@vger.kernel.org # v5.7+
Fixes: 4ebf9693 ("media: venus: introduce core selection")
Tested-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Mansur Alisha Shaik <mansur@codeaurora.org>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>