radv: handle exporting view index to fragment shader. (v1.1)

radv: handle exporting view index to fragment shader. (v1.1)

The fragment shader was trying to read this, but nothing
was exporting it from the vertex shader. This handles
it like the prim id export.

Fixes:
dEQP-VK.multiview.secondary_cmd_buffer.*
dEQP-VK.multiview.index.fragment_shader.*

v1.1: updated to use 0x1 (Samuel)

Fixes: e3265c10c89 (radv: Implement multiview draws.)
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>