drm/vc4: Fix YUV plane handling when planes are in different buffers

YUV images can either be presented as one allocation with offsets
for the different planes, or multiple allocations with 0 offsets.

The driver only ever calls drm_fb_[dma|cma]_get_gem_obj with plane
index 0, therefore any application using the second approach was
incorrectly rendered.

Correctly determine the address for each plane, removing the
assumption that the base address is the same for each.

Fixes: fc04023f

 ("drm/vc4: Add support for YUV planes.")
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>