llvmpipe: improve rasterization discard logic

llvmpipe: improve rasterization discard logic

This unifies the explicit rasterization discard as well as the implicit
rasterization disabled logic (which we need for another state tracker),
which really should do the exact same thing.
We'll now toss out the prims early on in setup with (implicit or
explicit) discard, rather than do setup and binning with them, which
was entirely pointless.
(We should eventually get rid of implicit discard, which should also
enable us to discard stuff already in draw, hence draw would be
able to skip the pointless clip and fallback stages in this case.)
We still need separate logic for only null ps - this is not the same
as rasterization discard. But simplify the logic there and don't count
primitives simply when there's an empty fs, regardless of depth/stencil
tests, which seems perfectly acceptable by d3d10.
While here, also fix statistics for primitives if face culling is
enabled.
No piglit changes.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>