r600g: Implement gpu_shader5 textureGather

r600g: Implement gpu_shader5 textureGather

Adds 0-3 textureGather component selection and non-constant offsets

Caveat: 0 and 1 texture swizzles only work if textureGather component
select is 3 or a component that does not exist in the sampler texture
format. This is a hardware limitation, any other value returns
128/255=0.501961 for both 0 and 1.

Passes all textureGather piglit tests on radeon 6670, except for those
using 0/1 texture swizzles due to aforementioned reason.

Signed-off-by: Glenn Kennard <glenn.kennard@gmail.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>