st/mesa: only set up sampler compare mode for depth textures

st/mesa: only set up sampler compare mode for depth textures

The GL_ARB_shadow spec says the shadow compare mode should have no
effect when sampling a color texture.  As it was, it was up to
drivers to check for that (softpipe, llvmpipe, svga and probably
the rest don't do that).  Note: it looks like DX10 allows shadow
compare with some non-depth formats, so this case really should be
handled in the state tracker.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>