d3d11h264dec: Keep track of actually configured DPB size

... instead of the largest we ever seen.
Note that d3d11h264dec element holds previously configured DPB size
for later decoder object re-open decision.

This is to fix below case:
1) Initial SPS, required DPB size is 6
  - decoder object is opened with DPB size 6
  - max_dpb_size is now 6
2) SPS update with resolution change, required DPB size is 1
  - decoder object is re-opened with DPB size 1
  - max_dpb_size should be updated to 1, but it didn't happen (BUG)
3) SPS update without resolution change, only required DPB size is updated to 6
  - decoder object should be re-opened but didn't happen
    because we didn't update max_dpb_size at 2).

Part-of: <gstreamer/gst-plugins-bad!2057>
29 jobs for !2057 with backport-d3d11h264dec in 26 minutes and 39 seconds (queued for 2 seconds)
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  Build Docker
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gst-ios-14.3
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gst-macos-11.1
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build msys2
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build static fedora x86_64

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  Integrate
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