freedreno/a6xx: samplerBuffer fixes

freedreno/a6xx: samplerBuffer fixes

Use the 'UNK31' bit (which should probably be called 'BUFFER') for
samplerBuffer case, which increases the size of supported buffer
texture beyond 2^15 elements.

Also need to fix the 2nd coord injected to handle the tex instructions
that take integer coords.

Fixes dEQP-GLES31.functional.texture.texture_buffer.render.as_fragment_texture.buffer_size_131071
and similar

Signed-off-by: Rob Clark <robdclark@gmail.com>