fb: correct invert_area image calculation

Fix the wrong calculation for inverting implemented by
commit 70685875059f
("fb: cfb: support inverting with coordinates that do not align
with the tile")

Fixed not enough consideration when the drawing area height
is eight lines or less.
Simplify to XOR calculation.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>
1 file changed