vidc_3x: Query Entropy property only for H264 format

V4L2_CID_MPEG_VIDEO_H264_ENTROPY_MODE is valid for h264
format only. No need to call get_prop of this for other formats.

Change-Id: I620ca1a799479c42399e683d9cc6f187ff7c3626
Signed-off-by: Vasantha Balla <vballa@codeaurora.org>