i965: Use swizzles to force R, G, and B to 0.0 for ALPHA textures.

i965: Use swizzles to force R, G, and B to 0.0 for ALPHA textures.

Similar to the previous commit, we may be using a texture with actual RGBA
storage for the GL_ALPHA format, so force the color values to 0.0.

This commit fixes the following piglit (sub) tests:

	EXT_texture_snorm/fbo-blending-formats
		GL_ALPHA16_SNORM
	        GL_ALPHA8_SNORM
		GL_ALPHA_SNORM

Note: Haswell bypasses this swizzle code, so may require an independent fix
for this bug.

Reviewed-by: Eric Anholt <eric@anholt.net>