drm/i915: Use pipe config in sprite code

Rather than dig up the pipe source size from crtc->mode, use
intel_crtc->config.requested_mode.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>