radv: Use the correct channel for alpha in resolve srgb conversion.
The argument here is a bitmask, so the old code selected .xy, which got silently truncated to .x when constructing the vec4 from components, instead of using .w. Fixes: 588185eb "radv/meta: add srgb conversion to end of resolve shader." Reviewed-by:Dave Airlie <airlied@redhat.com>