freedreno/ir3: fix RA issue with fanin

freedreno/ir3: fix RA issue with fanin

The fanin source could be grouped, for example with shaders like:

    VERT
    DCL IN[0]
    DCL IN[1]
    DCL OUT[0], POSITION
    DCL OUT[1], GENERIC[9]
    DCL SAMP[0]
    DCL SVIEW[0], 2D, FLOAT
    DCL TEMP[0], LOCAL
      0: MOV TEMP[0].xy, IN[1].xyyy
      1: MOV TEMP[0].w, IN[1].wwww
      2: TXF TEMP[0], TEMP[0], SAMP[0], 2D
      3: MOV OUT[1], TEMP[0]
      4: MOV OUT[0], IN[0]
      5: END

The second arg to the isaml is IN[1].w, so we need to look at the fanin
source to get the correct offset.

Signed-off-by: Rob Clark <robclark@freedesktop.org>