mesa/st: ignore StencilSampling if stencil not part of the format
mesa/st: ignore StencilSampling if stencil not part of the format
This avoids hitting assert in debug builds and incorrect rendering
on release in case GL_DEPTH_STENCIL_TEXTURE_MODE has been set to
GL_STENCIL_INDEX with a texture that does not have stencil.
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>