i965: Rename nr_depth_regs to nr_payload_regs.

i965: Rename nr_depth_regs to nr_payload_regs.

Only 8 out of the up to 13 regs are for source/dest depth, so the name
wasn't particularly appropriate.  Note that this doesn't count the
constant or URB payload regs.  Also, don't pre-divide by 2, so it's
actually a number of registers.