i965/gs: Fix incorrect numbering of DWORDs in 3DSTATE_GS

In commit 247f90c77e8f3894e963d796628246ba0bde27b5 (i965/gs: Set
control data header size/format appropriately for EndPrimitive()), I
incorrectly numbered the DWORDs in the 3DSTATE_GS command starting
from 1 instead of starting from 0.  This caused the control data
format to be programmed into the wrong DWORD, resulting in corruption
in some geometry shaders that used an output type of points.

This patch numbers the DWORDs starting from 0, as we do for all other
commands, which causes the control data format to be programmed into
the correct DWORD.