intel/batch-decoder: fix vertex buffer size calculation for gen<8

intel/batch-decoder: fix vertex buffer size calculation for gen<8

It should be incremented by one according to
how it is calculated by 'emit_vertex_buffer_state':
  "\#if GEN_GEN < 8
      .BufferAccessType = step_rate ? INSTANCEDATA : VERTEXDATA,
      .InstanceDataStepRate = step_rate,
   \#if GEN_GEN >= 5
      .EndAddress = ro_bo(bo, end_offset - 1),
   \#endif
   \#endif"

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109449
Signed-off-by: Andrii Simiklit <andrii.simiklit@globallogic.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>