i965: Delete the brw_vue_program_key::userclip_active flag.

i965: Delete the brw_vue_program_key::userclip_active flag.

There are two uses of this flag.

The primary use is checking whether we need to emit code to convert
legacy gl_ClipVertex/gl_Position clipping to clip distances.  In this
case, we also have to upload the clip planes as uniforms, which means
setting nr_userclip_plane_consts to a positive value.  Checking if it's
> 0 works for detecting this case.

Gen4-5 also wants to know whether we're doing clipping at all, so it can
emit user clip flags.  Checking if output_reg[VARYING_SLOT_CLIP_DIST0]
is set to a real register suffices for this.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>