drm/i915: Skip wm/ddb readout for disabled pipes

The stuff programmed into the wm/ddb registers of planes
on disabled pipes doesn't matter. So during readout just
leave our software state tracking for those zeroed.

This should avoid us trying too hard to clean up after
whatever mess the VBIOS/GOP left in there. The actual
hardware state will get cleaned up if/when we enable
the pipe anyway.

Cc: stable@vger.kernel.org
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/5711
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220617195948.24007-1-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>