gallium/u_transfer_helper: Don't leak a reference to the resource.

We pipe_resource_reference when handling transfers in map, we need to
do a corresponding unreference in unmap.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
10 jobs for !1199 with transfer-leaks in 8 minutes and 42 seconds (queued for 6 seconds)
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Status Job ID Name Coverage
  Containers Build
passed debian #388280

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  Build+Test
passed meson-clang #388282

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passed meson-clover #388286

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passed meson-main #388285

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passed meson-swr-glvnd #388281

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passed meson-vulkan #388287

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passed scons-llvm #388289

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passed scons-nollvm #388288

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passed scons-swr #388283

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passed scons-win64 #388284

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