st/mesa: fallback to 16-bit floating point formats for ARB_texture_float.

There exists a set of intel hw (gen4) that can't do filtering on 32-bit float formats, however it exposes floating point format on GL 2.1

The main mesa format picker and intel i965 driver support these fallback paths, and state the GL2.1 spec allows for such things.

Extend the state tracker format code to allow 16-bit float fallbacks.