drm/radeon/kms/r100: fix bug in CS parser

The first dword of PACKET3_3D_DRAW_IMMD maps to
SE_VTX_FMT so the vertex size is part of the draw
packet.

This patch fixes a possible case where you have a
command buffer that does not contain SE_VTX_FMT
register write, but does contain PACKET3_3D_DRAW_IMMD.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>