i965/vs: add support for ir_txf_ms on Gen6+

i965/vs: 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`. This takes an additional MCS parameter to support
compressed multisample surfaces, but we're not enabling them for
multisample textures for now, so it's always ignored and can be safely
omitted.

V2: Reworked completely, added support for Gen7.
V3: - Use new sample_index, sample_index_type rather than reusing lod
    - Clarify commit message.
V4: - Fix comment style

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Eric Anholt <eric@anholt.net>