drm/i915: Don't do the WM0->WM1 copy w/a if WM1 is already enabled

drm/i915: Don't do the WM0->WM1 copy w/a if WM1 is already enabled

Due to a workaround we have to make sure the WM1 watermarks block/lines
values are sensible even when WM1 is disabled. To that end we copy those
values from WM0.

However since we now keep each wm level enabled on a per-plane basis
it doesn't seem necessary to do that copy when we already have an
enabled WM1 on the current plane. That is, we might be in a situation
where another plane can only do WM0 (and thus needs the copy) but
the current plane's WM1 is still perfectly valid (ie. fits into the
current DDB allocation).

Skipping the copy could avoid reprogramming the plane's registers
needlessly in some cases.

Fixes: a301cb0fca2d ("drm/i915: Keep plane watermarks enabled more aggressively")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230131002127.29305-1-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>