msm: venc: do not update operating rate during INT_MAX

INT_MAX is an interface provided to client to configure
video session in TURBO mode. When client configures the
operating rate as INT_MAX, the rate at which the frames
arrive at video driver does not change, so there is no
need to update the operating rate.
If the operating rate is updated, macroblocks/s for the
session goes high. As a result, the session gets rejected
as the clock is not sufficient for high mbs/s.

Change-Id: I852c4ea8b30c736b3596c3b9bd5960b68904e359
Signed-off-by: Vikash Garodia <vgarodia@codeaurora.org>