winsys/virgl: Initialize value to silence valgrind

winsys/virgl: Initialize value to silence valgrind

Silences:

  Conditional jump or move depends on uninitialised value(s)
  at 0xB72F2C0: virgl_drm_winsys_create (virgl_drm_winsys.c:854)
  by 0xB72F2C0: virgl_drm_screen_create (virgl_drm_winsys.c:926)
  by 0xB21C885: pipe_virgl_create_screen (drm_helper.h:275)
  by 0xB7201F0: pipe_loader_create_screen (pipe_loader.c:137)
  by 0xB639C91: dri2_init_screen (dri2.c:2112)
  by 0xB634F68: driCreateNewScreen2 (dri_util.c:153)
  by 0x63023E6: dri3_create_screen (dri3_glx.c:893)
  by 0x62D35BD: AllocAndFetchScreenConfigs (glxext.c:820)
  by 0x62D35BD: __glXInitialize (glxext.c:946)
  by 0x62CECB3: GetGLXPrivScreenConfig (glxcmds.c:174)
  by 0x62CF69C: glXQueryExtensionsString (glxcmds.c:1304)
  by 0x60AA7D9: ??? (in /usr/lib/x86_64-linux-gnu/libwaffle-1.so.0.5.2)
  by 0x4F81450: wfl_checked_display_connect (piglit-util-waffle.h:74)
  by 0x4F829E0: piglit_wfl_framework_init (piglit_wfl_framework.c:627)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>