llvmpipe: get rid of depth swizzling.

llvmpipe: get rid of depth swizzling.

Eliminating this we no longer need to copy between linear and swizzled layout.
This is probably not quite ideal since it's a bit more work for now, could do
some optimizations by moving depth testing outside the fragment shader loop
(but tricky for early depth test as we don't have neither the mask nor the
interpolated z in the right order handy).
The large amount of tile/untile code is no longer needed will be deleted
in next commit.
No piglit regressions.
v2: change a forgotten LAYOUT_NONE to LAYOUT_LINEAR.
v3: fix (bogus) uninitialized variable warnings, add comments, fix a bad type

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