i965/fs: add support for ir_txf_ms on Gen6+

On Gen6, lower this to `ld` with lod=0 and an extra sample_index
parameter.

On Gen7, use `ld2dms`. We don't support CMS yet for multisample
textures, so we just hardcode MCS=0. This is ignored for IMS and UMS
surfaces.

Note: If we do end up emitting specialized shaders based on the MSAA
layout, we can emit a slightly shorter message here in the UMS case.

Note: According to the PRM, `ld2dms` takes one more parameter, lod.
However, it's always zero, and including it would make the message too
long for SIMD16, so we just omit it.

V2: Reworked completely, added support for Gen7.
V3: - Introduce sample_index parameter rather than reusing lod
    - Removed spurious whitespace change
    - Clarify commit message
V4: - Fix comment style
    - Emit SHADER_OPCODE_TXF_MS on Gen6. This was benignly wrong since
      it lowers to `ld` anyway on this gen, but still wrong.

Signed-off-by: Chris Forbes <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>

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