drm/plane: Clarify our expectations for src/dst rectangles

The rectangles are usually clipped, but it can be useful to have
them unclipped, for example for cursor planes.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[mlankhorst: Change cursor plane to hardware performing clipping. (Ville)
             Fix dst description that went missing.]
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191010112918.15724-1-maarten.lankhorst@linux.intel.com