drm/i915: Remove misleading CSR firmware loading docs

I forgot to remove these when reworking the firmware loading sequence
last year. The new sequence is that we load firmware, and if it's not
there we entirely (and permanently) fail dmc setup.

Reported-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468505704-17391-1-git-send-email-daniel.vetter@ffwll.ch