anv/formats: Set the swizzle to RGB1 when using an RGBA format to fake RGB

anv/formats: Set the swizzle to RGB1 when using an RGBA format to fake RGB

This way we get correct sampling from RGB formats that are faked as RGBA.
This should also cause it to disable rendering and blending on those
formats.  We should be able to render to them and, on Broadwell and above,
we can blend on them with work-arounds.  However, we'll add support for
that more properly later when it's deemed useful.  For now, disabling
rendering and blending should be safe.