i965/vec4/gs: Stop munging the ATTR containing gl_PointSize.

i965/vec4/gs: Stop munging the ATTR containing gl_PointSize.

gl_PointSize is delivered in the .w component of the VUE header, while
the language expects it to be a float (and thus in the .x component).

Previously, we emitted MOVs to copy it over to the .x component.
But this is silly - we can just use a .wwww swizzle and access it
without copying anything or clobbering the value stored at .x
(which admittedly is useless).

Removes the last use of ATTR destinations.

v2: Use BRW_SWIZZLE_WWWW, not SWIZZLE_WWWW (caught by GCC).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>