mesa: begin reducing memory used by display lists

mesa: begin reducing memory used by display lists

This is a first step in reducing memory used by display lists on
64-bit systems.  On 64-bit systems, the gl_dlist_node union type
is 8 bytes because of the 'data' and 'next' fields.  This causes
every display list node/token to occupy 8 bytes instead of 4 as
originally designed.  This basically doubles the memory used by
some display lists on 64-bit systems.

The fix is to remove the 64-bit 'data' and 'next' pointer fields
from the union and instead store them as a pair of 32-bit values.
Easily done with a few helper functions.

The next patch will take care of the 'next' field.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>