radeonsi: fix fmask expand compute shader

radeonsi: fix fmask expand compute shader

'coord' variable was using TGSI_WRITEMASK_XYZ so subsequent uses of
TGSI_WRITEMASK_W were dropped.
The result for a 2 samples program was:

  0: UMAD TEMP[0].xy, SV[1].xyyy, IMM[0].xxxx, SV[0].xyyy
  1: STORE IMAGE[0], TEMP[0], TEMP[1], RESTRICT, 2D_MSAA
  2: STORE IMAGE[0], TEMP[0], TEMP[2], RESTRICT, 2D_MSAA
  3: END

instead of the expected:

  0: UMAD TEMP[0].xy, SV[1].xyyy, IMM[0].xxxx, SV[0].xyyy
  1: MOV TEMP[0].w, IMM[0].yyyy
  2: LOAD TEMP[1], IMAGE[0], TEMP[0], RESTRICT, 2D_MSAA
  3: MOV TEMP[0].w, IMM[0].zzzz
  4: LOAD TEMP[2], IMAGE[0], TEMP[0], RESTRICT, 2D_MSAA
  5: MOV TEMP[0].w, IMM[0].yyyy
  6: STORE IMAGE[0], TEMP[0], TEMP[1], RESTRICT, 2D_MSAA
  7: MOV TEMP[0].w, IMM[0].zzzz
  8: STORE IMAGE[0], TEMP[0], TEMP[2], RESTRICT, 2D_MSAA
  9: END

This fixes half of https://gitlab.freedesktop.org/mesa/mesa/issues/2248

Fixes: 095a58204d9 ("radeonsi: expand FMASK before MSAA image stores are used")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3174>