msm: vidc: Use bitrate to scale clocks for decoder

The Venus core's ability to handle bitrate depends on
the clock frequency it operates at. If we get spikes in
bitrate, we should scale the core clock to a higher frequency,
at which the Venus core can decode the frame within the
required time. This change calculates the instantaneous
bitrate for each input frame and scales the clock accordingly.
Without this change we will see frame drops whenever there are
spikes in bitrate.

Bitrate can be switched on and off using the following
debugfs handle - /d/msm_vidc/bitrate_clock_scaling

Change-Id: I772b132c6b8b39ae186c69b8caec1b95d108c23b
Signed-off-by: Arun Menon <avmenon@codeaurora.org>