GPUUploadManager: fix potential issue with large uploads #4149

Job	Run time
Linux -> Pre-checks	17s
Linux -> Clang-NO_GLSLANG-x64, Debug	8m 15s
Linux -> Clang-x64, Release	13m 26s
Linux -> Clang-NO_GLSLANG-NO_HLSL-x64, Release	6m 26s
Linux -> GCC-x64, Debug	12m 25s
Linux -> Clang-x64, Debug	13m 18s
Linux -> GCC-x64, Release	10m 35s
	1h 4m 42s

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