i965/gs: Fix up gl_PointSize input swizzling for DUAL_INSTANCED gs.

i965/gs: Fix up gl_PointSize input swizzling for DUAL_INSTANCED gs.

Geometry shaders that run in "DUAL_INSTANCED" mode store their inputs
in vec4's.  This means that when compiling gl_PointSize input
swizzling (a MOV instruction which uses a geometry shader input as
both source and destination), we need to do two things:

- Set force_writemask_all to ensure that the MOV happens regardless of
  which channels are enabled.

- Set the source register region to <4;4,1> (instead of <0;4,1> to
  satisfy register region restrictions.

v2: move the source register region fixup to the top of
vec4_generator::generate_vec4_instruction(), so that it applies to all
instructions rather than just MOV.

Reviewed-by: Eric Anholt <eric@anholt.net>