Restrict the search of drl idx to 0-2 in speed >= 2

[kslu-aom]

This restricts the default encoder behavior. The existing encode flag --max-drl-refmvs can still overwrite the search range.

Evaluation based on fe1bfde with 33 frames at speed 2:

+---------+--------+--------+--------+--------+----------+----------+
| Summary |   Y    |   U    |   V    |  YUV   | Enc-time | Dec-time |
+---------+--------+--------+--------+--------+----------+----------+
| A1      | +0.16% | -0.01% | +0.14% | +0.15% |  95.2%   | 100.1%   |
| A2      | +0.18% | -0.16% | -0.11% | +0.16% |  92.7%   |  97.8%   |
+---------+--------+--------+--------+--------+----------+----------+

[kslu-aom]

The speedup-loss tradeoff is not good after rebase due to conflicts with recently added speed features. Marked as draft. I will rework this patch.

[yeqing-wu]

@kslu-aom, I’ve done this before, and at that time, the tradeoff wasn’t good either. However, it appears to be acceptable after the commit of this PR, but it’s not good again on the latest ToT.