swr/rast: more flexible max attribute slots

swr/rast: more flexible max attribute slots

Ability to allocate space for an arbitrary number (at compile time)
of positions in the vertex layout.

Removes KNOB_NUM_ATTRIBUTES from knobs.h, replaces the VTX slot
number #defines with the SWR_VTX_SLOTS enum (which contains
replacement for NUM_ATTRIBUTES: SWR_VTX_NUM_SLOTS)

Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>