drm/vc4: plane: Fix margin calculations for the right/bottom edges

The current plane margin calculation code clips the right and bottom
edges of the range based using the left and top margins.

This is obviously wrong, so let's fix it.

Fixes: 666e7358 ("drm/vc4: Take margin setup into account when updating planes")
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://lore.kernel.org/r/20220613144800.326124-6-maxime@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>