i965: allow 8 user clip planes on CTG+

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.
    - Fix clip flag masks in the clip unit state and in the shader
      prolog
    - Move this to the end of the series for less breakage.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Paul Berry <stereotype441@gmail.com>