i965: Fix component mask and varying_to_slot mapping for gl_ViewportIndex

i965: Fix component mask and varying_to_slot mapping for gl_ViewportIndex

gl_ViewportIndex doesn't get its own varying slot. It is stored
in VARYING_SLOT_PSIZ.z. This patch fixes the issue for both gen7
and gen8 because gen7_upload_3dstate_so_decl_list() is shared
between them.

Fixes failures in OpenGL Khronos CTS test transform_feedback_builtins.
Makes new piglit test glsl-1.50-transform-feedback-builtins pass for
'gl_ViewportIndex'.

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>