i965: Use 4 bits to store nr_userclip in brw_clip.h.

i965: Use 4 bits to store nr_userclip in brw_clip.h.

Since the i965 driver supports 8 clipping planes now, we need 4 bits
to store the number of user clipping planes, not 3.

In theory this isn't strictly necessary, since brw_clip.h is only used
on pre-GEN6, and pre-GEN6 only advertises support for 6 clipping
planes, but it seems wise to err on the safe side.

In the process I removed the pad0 element of struct
brw_clip_prog_key--it doesn't seem necessary because the compiler
automatically inserts padding if needed.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>