i965: allow 8 user clip planes on CTG+

There's no need to use a clip flag for NEGW on these gens, so
no reason we can't just enable 8 planes.

V2: Bump (and document!) MAX_VERTS in the clip code.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/Gen4-5: allocate VUE slots for clipdistance if user clipping is enabled

Also adjust the SF URB read offset to account for there being two
additional slots of stuff it doesn't care about.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/fs: Gen4/5: Fix inconsistency in the VUE map.

The SF never gets to see the clip distances. If we didn't ignore them
here, we got confused about the correct offsets for any varyings laid
out after them. (user-defined varyings would break, but gl_FrontColor
etc worked properly).

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/vs: Do legacy clip lowering earlier

We need to produce clip flags for the vertex header on Gen4/5, so
clip plane lowering has to be done before we try to emit the flags/psiz
attribute.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/clip: Enable interpolation of clip distances

Previously we had disabled interpolation of the clip distances as a
special case, since they were unused.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/Gen4-5: Set clip flags from clip distances

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/clip: Track offset into the vertex for clipdistance

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i956/clip: push dp4 into load_clip_distance

Soon the dp4 is only going to be used for fixed clip planes.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/clip: Use clip distances for all user clipping

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/clip: remove spurious clipvertex param

Nothing in the clipper uses gl_ClipVertex any more, so we don't care
where it is.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>