i965: Add BRW_SURFACEFORMAT_R32G32_FLOAT_LD, required for IVB gather4 w/a

gather4 GREEN channel against a surface with format R32G32_FLOAT doesn't work
correctly on IVB. w/a from bspec:

   - use R32G32_FLOAT_LD = 0x97 instead, for gather4 only.
   - select BLUE channel to read GREEN

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
make room in the binding table for a full alternate set of surface_states
i965: Emit a second set of SURFACE_STATE for gather4 from textures.

This allows us to use a different surface format for gather4, which is
required for R32G32_FLOAT to work on Gen7.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
emit reference to gather surfaces when gather in use
i965: w/a for gather4 green RG32F