sunxi: Possible G2D kernel bug

G2D seems to break if a zero value of src_rect.w or src_rect.h is
provided to g2d_blit, g2d_fill or g2d_stretchblit.
para.src_rect.w == 0 or para.src_rect.h == 0 results in trying to write
-1 toG2D_DMA0_SIZE_REG register within the mixer_* functions, e.g. here
https://github.com/linux-sunxi/linux-sunxi/blob/sunxi-3.4/drivers/char/sunxi_g2d/g2d_bsp.c#L682

The following patch catches these zero value cases.

Signed-off-by: Andreas Baierl <list@imkreisrum.de>
Signed-off-by: Henrik Nordstrom <henrik@henriknordstrom.net>