msm: vidc: Increase max encoder bitrate to 300 mbps

The more recent iterations of Venus support encoding at 300 mbps, so
there's no reason for the driver to artifically cap at 160 mbps.

Change-Id: I59e42bf89a8127085df40d4f71bf195e554741c2
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>