Extend ref_mv_idx search pruning to speed 1-5

[YixinDu1]

Enable the DRL ref_mv_idx search pruning for speeds 1-5.

Anchor: 7837c40
Test condition: CTC, RA, 33 frames

RA, speed 4:

+------------+-------+-------+-------+-------+------+
| Class      |     Y |    Cb |    Cr |  wAvg | Enc% |
+------------+-------+-------+-------+-------+------+
| A1         |  0.08 |  0.06 |  0.15 |  0.09 | 97.8 |
| A2         |  0.10 |  0.15 |  0.06 |  0.09 | 97.3 |
+------------+-------+-------+-------+-------+------+
| Avg w/o B2 |  0.09 |  0.15 |  0.26 |  0.10 | 97.3 |
+------------+-------+-------+-------+-------+------+

RA, speed 1:

+------------+-------+-------+-------+-------+------+
| Class      |     Y |    Cb |    Cr |  wAvg | Enc% |
+------------+-------+-------+-------+-------+------+
| A1         |  0.06 |  0.22 | -0.01 |  0.07 | 97.3 |
| A2         |  0.07 |  0.26 |  0.08 |  0.08 | 96.7 |
+------------+-------+-------+-------+-------+------+
| Avg w/o B2 |  0.05 |  0.19 |  0.10 |  0.06 | 96.6 |
+------------+-------+-------+-------+-------+------+

STATS_CHANGED