i965: Use R16G16B16A16_FLOAT for 3-component half-float.

i965: Use R16G16B16A16_FLOAT for 3-component half-float.

The RGBX version isn't supported as a vertex input type, but since we
force the last channel's value anyway, this should be fine.  The only
potential risk I see is in the limiter on VBO reads past the end of
the buffer forcing the whole vertex to 0 when the A channel lands past
the end.

Fixes piglit draw-vertices-half-float.