drm/plane-helper: Fix primary plane scaling check

The src_w / src_h parameters to update_plane include a subpixel offset;
we need to shift off the subpixel bits before comparing to CRTC size
when checking for primary plane scaling.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>