r300g: add generating texture coordinates for point sprites

r300g: add generating texture coordinates for point sprites

[airlied -
	Convert sprite coord index to a per-coord enable bit
	set the rasteriser block up correctly for point sprites.

The inputs to the RS hw block change for sprite coords, so fix them up
properly - this fixes piglit point-sprite test.
]

Signed-off-by: Dave Airlie <airlied@redhat.com>