lavapipe: use the correct value for dynamic render resolve attachment indexing

subpass->color_count is (obviously) not set yet, so this would just clobber the color attachments any time resolves were used

Fixes: 8a6160a3 ("lavapipe: VK_KHR_dynamic_rendering")

fixes: dEQP-VK.draw.dynamic_rendering.multiple_interpolation.structured.with_sample_decoration.4_samples