We don't need to look at the framebuffer state and record how many color
buffers to write. Instead, we can deduce which color buffers are enabled
from spi_shader_col_format, which already does the right thing.
So PS epilogs only need a single bool flag that determines whether all
enabled color buffers should be written.
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32910>
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