i965: Don't emit MOVs with undefined registers for Gen4 point clipping.

i965: Don't emit MOVs with undefined registers for Gen4 point clipping.

Gen4 point clipping calls brw_clip_tri_alloc_regs with nr_verts == 0,
which means that c->reg.vertex[] isn't initialized.  It then emits MOVs
to stomp components of those uninitialized registers to 0.

This started causing assertions after Matt's recent series, when those
uninitialized registers started getting BRW_REGISTER_TYPE_NF, which
definitely doesn't exist on Gen4-5.

Reviewed-by: Matt Turner <mattst88@gmail.com>