i965: Add message descriptor bit definitions for pixel interpolator

These got lost in the big brw_inst shakeup.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/disasm: add support for pixel interpolator messages

V3: Rework for brw_inst changes

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965: add low-level support for send to pixel interpolator

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/fs: add generator support for pixel interpolator query

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/fs: Skip channel expressions splitting for interpolation

The backend will have to do a message send, so we want to keep these in
one piece, just like texture ops.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/fs: add support for ir_*_interpolate_at_* expressions

SIMD8-only for now.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>