media: venus: fix vpp frequency calculation for decoder

In existing video driver implementation vpp frequency calculation in
calculate_inst_freq() is always zero because the value of vpp_freq_per_mb
is always zero for decoder.

Fixed this by correcting the calculating the vpp frequency calculation for
decoder.

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>