radv: Fix depth-only-with-discard when epilogs are used.

radv: Fix depth-only-with-discard when epilogs are used.

For a depth-only-with-discard pipeline, spi_shader_col_format needs to be
fixed up to a single channel export, or otherwise discard will not work.

Since col_format can change depending on the dynamic state, precompute the
need for this workaround on pipeline creation and apply it when emitting
prolog states.

Fixes: eb07a11b8f4 ("radv: add support for compiling PS epilogs on-demand")
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20704>