drm/i915: Don't skip ddb allocation if data_rate==0

data_rate==0 no longer means a plane is disabled, it could
also mean we want to use the minimum ddb allocation for it.
Hence we can't bail out early during ddb allocation or
else we'll simply forget to allocate any ddb for such planes.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Fixes: 6a4d8cc6

 ("drm/i915: Don't allocate extra ddb during async flip for DG2")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220214105532.13049-2-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>