r600: update pipe_draw_info::restart_index only when primitive_restart is enabled

This avoids the use of uninitialized memory:

Conditional jump or move depends on uninitialised value(s)
   at 0x5EDBF87: r600_draw_vbo (r600_state_common.c:2232)
   by 0x5DEFA00: u_vbuf_draw_vbo (u_vbuf.c:1329)
   by 0x5D990EF: cso_multi_draw (cso_context.c:1445)
   by 0x59B7EF4: _mesa_draw_arrays (draw.c:1346)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
41 jobs for !10428 with fix-initialize-draw-info in 9 seconds (queued for 4 seconds)
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sanity

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aarch64 manual
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windows shell 1809 mesa manual
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aarch64
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ppc64el_build
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meson-testing-asan
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meson-vulkan
 
  Build Misc
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arm_test
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meson-android
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aarch64
meson-arm64
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aarch64
meson-arm64-asan
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aarch64
meson-arm64-build-test
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aarch64
meson-armhf
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virgl-gles2-on-gl
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virgl-gles31-on-gl 1/2
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virgl-gles31-on-gl 2/2
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virgl-gles3-on-gl
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virgl-traces