msm: vidc: ensure minimum bus vote during sessions

If all video sessions are closed but venus firmware is
not unloaded yet then power_on function is not ensuring
to vote for minimum bus frequency leading to bus transaction
failures. This change will ensure minimum bus voting in
all scenarios.

Change-Id: I55b3796ae5936f757e1c0f94257011f680976179
Signed-off-by: Maheshwar Ajja <majja@codeaurora.org>