winsys/amdgpu: extract adding a new buffer list entry into its own function
winsys/amdgpu: extract adding a new buffer list entry into its own function

While at it, try to be a little more robust in the face of memory allocation
failure.

Reviewed-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler