radeonsi: fix ps_uses_fbfetch value

radeonsi: fix ps_uses_fbfetch value

si_update_ps_colorbuf0_slot used blitter_running as a way to detect
recursive calls.
Unfortunately this catch too many cases; for instance a backtrace
like:
  #0 si_update_ps_colorbuf0_slot
  #1 si_set_framebuffer_state
  #2 do_blits
  [...]
  #5 si_blit
  #6 si_copy_region_with_blit

Would end-up not updating ps_uses_fbfetch; so if the new fb_state is
something like:
  cbufs = {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}, zsbuf = 0x55b8987545e0}

We can have ps_uses_fbfetch=true but cbufs[0] = NULL, which causes a
crash later in si_ps_key_update_framebuffer.

This commit fixes intermittent crashes in KHR-GL46.stencil_texturing.functional.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13550>