virgl: don't flush an empty range
Otherwise, the gl-1.0-long-dlist Piglit test crashes.
Fixes: db7757 ("virgl: modify how we handle GL_MAP_FLUSH_EXPLICIT_BIT") Reported by airlied@
Otherwise, the gl-1.0-long-dlist Piglit test crashes.
Fixes: db7757 ("virgl: modify how we handle GL_MAP_FLUSH_EXPLICIT_BIT") Reported by airlied@