radv: fix dynamic RT stack size with VGPR spilling

radv: fix dynamic RT stack size with VGPR spilling

VGPR spilling might cause VGPRs to be spilled at scratch offset 0, so we
can't use that.

fossil-db (Sienna Cichlid, Q2RTX and Control):
Totals from 4 (0.26% of 1524) affected shaders:
Instrs: 8734 -> 8737 (+0.03%)
CodeSize: 48492 -> 48504 (+0.02%)
Latency: 384375 -> 384369 (-0.00%)
InvThroughput: 256250 -> 256246 (-0.00%)
Copies: 1312 -> 1313 (+0.08%)
Branches: 256 -> 258 (+0.78%)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Konstantin Seurer <konstantin.seurer@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18541>