drm: Check plane src coordinates correctly during page flip for atomic drivers

Instead of relying on the old crtc-{x,y,mode} gunk, dig out the primary
plane coordinates from the plane state when checking them against the
new framebuffer during page flip.

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1444930802-8515-5-git-send-email-ville.syrjala@linux.intel.com


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