drm: mali-dp: Fix smart layer not going to composition

Use rectangle 1 as a generic plane. Existing code already sets the smart
layer bounding box size + offset. The rectangles' offsets are relative
to the bounding box, so there is no need to set R1's offset (reset value
is 0), just its size which is the same as the bounding box.

Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>