i965: Use {point_sprite,flat}_enable variable names instead of dw*.

i965: Use {point_sprite,flat}_enable variable names instead of dw*.

Calling the local variables flat_enable and point_sprite_enable is
clearer than dw16 and such.  It also matches the names used in
calculate_attr_overrides, which computes them.

v2: Add /* dw16 */ and /* dw10 */ comments, requested by Jordan.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>