[DX12 Enc] Rely on BRC reconfigure for AV1 per-frame-qp encoding.

base_q_idx is not intented for configuring per-frame-qp. Drivers
basically first overlook the BRC configuration. At the same time
we configure base_q_idx in picture param to make sure if some IHV
drivers are also looking at it.

Bug: 40275246, 430338394
Change-Id: I17b7e16916bd2ed49ce0476574ddd9fac790d822
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6963006
Reviewed-by: Eugene Zemtsov <eugene@chromium.org>
Commit-Queue: Jianlin Qiu <jianlin.qiu@intel.com>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1517669}