freedreno/headergen2: Fix compile warnings with CP_DRAW_INDIRECT_MULTI

freedreno/headergen2: Fix compile warnings with CP_DRAW_INDIRECT_MULTI

Using stripes to deal with the different packet layout variants resulted
in redefining "register" offsets with different values, so use "prefix"
to add a suffix to disambiguate.

  drivers/gpu/drm/msm/adreno/adreno_pm4.xml.h:1066: warning: "REG_A6XX_CP_DRAW_INDIRECT_MULTI_INDIRECT" redefined
   1066 | #define REG_A6XX_CP_DRAW_INDIRECT_MULTI_INDIRECT  0x00000006
        |
  drivers/gpu/drm/msm/adreno/adreno_pm4.xml.h:1057: note: this is the location of the previous definition
   1057 | #define REG_A6XX_CP_DRAW_INDIRECT_MULTI_INDIRECT  0x00000003
        |

(Admittedly it isn't really a "prefix" but that was the field in the
schema available to use, and REG_INDEXED_CP_DRAW_INDIRECT_MULTI_STRIDE
sounds somewhat more funny.)

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10944>