lavc/hevc_refs: Fix the logic of find_ref_idx()

lavc/hevc_refs: Fix the logic of find_ref_idx()

Currently find_ref_idx() would trigger 2 scans in DPB to find the
requested POC:
1. Firstly, ignore MSB of ref->poc and search for the requested POC;
2. Secondly, compare the entire ref->poc with requested POC;

For long term reference, we are able to only check LSB if MSB is not
presented(e.g. delta_poc_msb_present_flag == 0). However, for short
term reference, we should never ignore poc's MSB and it should be
kind of bit-exact. (Details in 8.3.2)

Otherwise this leads to decoding failures like:
[hevc @ 0x5638f4328600] Error constructing the frame RPS.
[hevc @ 0x5638f4328600] Error parsing NAL unit #2.
[hevc @ 0x5638f4338a80] Could not find ref with POC 21
Error while decoding stream #0:0: Invalid data found when processing input

Search the requested POC based on whether MSB is used, and avoid
the 2-times scan for DPB buffer. This benefits both native HEVC
decoder and integrated HW decoders.

Signed-off-by: Xu Guangxin <guangxin.xu@intel.com>
Signed-off-by: Linjie Fu <linjie.fu@intel.com>