r600: clean up the GS ring buffers when the context is destroyed

r600: clean up the GS ring buffers when the context is destroyed

This fixes two memory leaks reported by ASAN:

Direct leak of 248 byte(s) in 1 object(s) allocated from:
   in malloc (/usr/lib64/gcc/x86_64-pc-linux-gnu/7.3.0/libasan.so+0xdb880)
   in r600_alloc_buffer_struct ../../samba/mesa/src/gallium/drivers/r600/r600_buffer_common.c:578
   in r600_buffer_create ../../samba/mesa/src/gallium/drivers/r600/r600_buffer_common.c:600
   in r600_resource_create_common ../../samba/mesa/src/gallium/drivers/r600/r600_pipe_common.c:1265
   in r600_resource_create ../../samba/mesa/src/gallium/drivers/r600/r600_pipe.c:725
   in pipe_buffer_create ../../samba/mesa/src/gallium/auxiliary/util/u_inlines.h:291
   in update_gs_block_state ../../samba/mesa/src/gallium/drivers/r600/r600_state_common.c:1482

Direct leak of 248 byte(s) in 1 object(s) allocated from:
   in malloc (/usr/lib64/gcc/x86_64-pc-linux-gnu/7.3.0/libasan.so+0xdb880)
   in r600_alloc_buffer_struct ../../samba/mesa/src/gallium/drivers/r600/r600_buffer_common.c:578
   in r600_buffer_create ../../samba/mesa/src/gallium/drivers/r600/r600_buffer_common.c:600
   in r600_resource_create_common ../../samba/mesa/src/gallium/drivers/r600/r600_pipe_common.c:1265
   in r600_resource_create ../../samba/mesa/src/gallium/drivers/r600/r600_pipe.c:722
   in pipe_buffer_create ../../samba/mesa/src/gallium/auxiliary/util/u_inlines.h:291
   in update_gs_block_state ../../samba/mesa/src/gallium/drivers/r600/r600_state_common.c:1489

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Fixes: 1371d65a7fbd695d3516861fe733685569d890d0
  r600g: initial support for geometry shaders on evergreen (v2)
Reviewed-by: Roland Scheidegger <sroland@vmware.com>