gallium: add a a new cap for changing the TGSI TG4 instruction encoding

gallium: add a a new cap for changing the TGSI TG4 instruction encoding

Accessing the TG4 component via immediates in the llvmpipe backend is quite
messy (like really messy). Roland suggested we change the instruction encoding,
so introduce a cap to allow the component to be selected to be store in the
sampler swizzle, which should be otherwise unused.

I could probably switch all drivers over, but virgl would need some work that
I'd prefer not to rush it.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>