ac/nir: account for view index in the user sgpr allocation.

The view index user sgpr wasn't being accounted for properly,
this refactors out the code to decide if it's required and then
uses that info to account for it.

Fixes: 180c1b92 (ac/nir: Add shader support for multiviews.)
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 3153d742)
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

Conflicts:
	src/amd/common/ac_nir_to_llvm.c