drm/i915/wm: remove ILK+ nop funcs fallback

Disabling ILK+ watermarks on failure to read the watermark levels dates
back to 2010 and commit 7f8a8569 ("drm/i915: Add the support of
memory self-refresh on Ironlake"), with no explanations, and it's been
copied and modified from that ever since. Finally drop it.

If the value are actually zero, the ilk_compute_*_wm() functions should
handle it gracefully.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215141910.433043-5-jani.nikula@intel.com