sunxi: g2d: handle noop blits

Zero area blits are technically valid noops and are requested bay
libvdpau. Return 0 when blit area is zero without performing bogus
calculations.

This reverts commit 3303e27 but also catches the zero values
which were leading to failed calculations.

Signed-off-by: Michal Suchanek <hramrach@gmail.com>
Signed-off-by: Andreas Baierl <ichgeh@imkreisrum.de>