drm: vop2: Do not divide height twice for YUV

For the cbcr format, gt2 and gt4 are computed again after src_h has been
divided by vsub.

As src_h as already been divided by 2 before, introduce cbcr_src_h and
cbcr_src_w to keep a copy of those values to be used for cbcr gt2 and
gt4 computation.

This fixes yuv planes being unaligned vertically when down scaling to
1080 pixels from 2160.

Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
Fixes: 604be855 ("drm/rockchip: Add VOP2 driver")