Log Message:
MesaLib18: more patch cleanup

patch-configure.ac: configure test for HAVE_OPEN_MEMSTREAM
patch-src_amd_common_ac__debug.c: ditto
patch-src_gallium_drivers_radeonsi_si__state__shaders.c: ditto
patch-src_gallium_auxiliary_util_u__helpers.c: add comment
patch-src_util_u__thread.h: add comment
patch-src_egl_drivers_dri2_platform__drm.c: fix incomplete #ifdef
patch-src_mesa_tnl_t__pipeline.c: delete obsolete patch