media: venus: correct low power frequency calculation for encoder

In exististing implimentation, in min_loaded_core() for low_power
vpp frequency value is considering as vpp_freq instead of low_power_freq.
Fixed this by correcting vpp frequency calculation for encoder.

Fixes: 3cfe5815 (media: venus: Enable low power setting for encoder)
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>