aco/gfx11: export mrtz in discard early exit for non-color shaders

aco/gfx11: export mrtz in discard early exit for non-color shaders

If a shader doesn't export any color targets and instead only exports
mrtz, the discard early exit block should match.

Fixes artifacts on Lara in Rise of the Tomb Raider benchmark and hair in
The Witcher 3 (classic).

https://reviews.llvm.org/D128185

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Fixes: bc8da20dda6 ("aco: export MRT0 instead of NULL on GFX11")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20345>
(cherry picked from commit 192486b7aa5ef3b24516e16b0e219fad7032fb7c)