v3d: implement tile buffer color read intrinsic

v3d: implement tile buffer color read intrinsic

We will be emitting this intrinsic to signal TLB color loads when we implement
OpenGL logic operations, where we need to blend the fragment shader color
output with the existing color in the render target.

Per-sample TLB reads are not supported yet.

v2: fix the offset into the color_reads array (Eric).

Reviewed-by: Eric Anholt <eric@anholt.net>