msm: vidc: Support VUI timing info in VFR mode as well

In contrast to V4L2_CID_MPEG_VIDC_VIDEO_FRAME_RATE, inst->prop.fps is
updated (or is atleast expected to as framerate changes) in VFR mode.
Hence, always populate timescale from inst->prop.fps.

Change-Id: I4220a98af982cef8f25d019570e1f1fbb904d169
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>