msm: vidc: Fix NULL access in bw governor

If we get a video session whose frame size exceeds more
than the supported frame size in LUT, we return NULL and
access NULL in the governor crashing the device. Instead of
NULL we should be returning the highest value supported.

Change-Id: I1ce25e93a0168a2669e2502ea427153a25435efd
Signed-off-by: Arun Menon <avmenon@codeaurora.org>