drm/i915: Use pipe_src_w in overlay code

Replace the use of crtc->mode.h/vdisplay with the more appropriate
config->pipe_src_w/h.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1481131693-27993-6-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>