gallium: add texture gather support to gallium (v3)

gallium: add texture gather support to gallium (v3)

This adds support to gallium for a TG4 instruction,
and two CAPs. The first CAP is required for GL_ARB_texture_gather.

The second CAP is required to expose GL_ARB_gpu_shader5.

However so far we haven't found any hardware that natively
exposes the textureGatherOffsets feature from GL, so just
lower it for now. If hardware appears for this we can add
another CAP to allow TG4 to take 4 offsets.

v2: add component selection src and a cap to say
hw can do it. (st can use to help control
GL_ARB_gpu_shader5/GLSL 4.00). Add docs.

v3: rename to SM5, add docs.

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