r600g: fix possible issue with stencil mipmap rendering
r600g: fix possible issue with stencil mipmap rendering

Somehow I only hit this issue with my latest libdrm changes.
This won't be needed with DB texturing.

NOTE: This is a candidate for the 9.0 branch.

djreep81/glsl-optimizer,zz85/glsl-optimizer,zz85/glsl-optimizer,mcanthony/glsl-optimizer,dellis1972/glsl-optimizer,mcanthony/glsl-optimizer,tokyovigilante/glsl-optimizer,wolf96/glsl-optimizer,dellis1972/glsl-optimizer,benaadams/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,bkaradzic/glsl-optimizer,dellis1972/glsl-optimizer,benaadams/glsl-optimizer,tokyovigilante/glsl-optimizer,zeux/glsl-optimizer,mapbox/glsl-optimizer,mapbox/glsl-optimizer,zeux/glsl-optimizer,metora/MesaGLSLCompiler,mapbox/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,tokyovigilante/glsl-optimizer,bkaradzic/glsl-optimizer,mcanthony/glsl-optimizer,jbarczak/glsl-optimizer,djreep81/glsl-optimizer,wolf96/glsl-optimizer,jbarczak/glsl-optimizer,djreep81/glsl-optimizer,benaadams/glsl-optimizer,djreep81/glsl-optimizer,mapbox/glsl-optimizer,wolf96/glsl-optimizer,mcanthony/glsl-optimizer,dellis1972/glsl-optimizer,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,zeux/glsl-optimizer,metora/MesaGLSLCompiler,benaadams/glsl-optimizer,mcanthony/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,wolf96/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,wolf96/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,metora/MesaGLSLCompiler,dellis1972/glsl-optimizer,benaadams/glsl-optimizer,mapbox/glsl-optimizer