drm/i915/display: Return error from dbuf allocation failure

drivers/gpu/drm/i915/intel_pm.c:5920 skl_ddb_add_affected_pipes() error: uninitialized symbol 'ret'.

Fixes: 3cf43cdc ("drm/i915: Introduce proper dbuf state")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200516190940.12675-1-chris@chris-wilson.co.uk