gpu: ipu-v3: Fix U/V offset macros for planar 4:2:0

The U and V offset macros for planar 4:2:0 (U_OFFSET, V_OFFSET, and
UV_OFFSET), are not correct. The height component to the offset was
calculated as:

(pix->width * y / 4)

But this does not produce correct offsets for odd values of y (luma
line #). The luma line # must be decimated by two to produce the
correct U/V line #, so the correct formula is:

(pix->width * (y / 2) / 2)

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>