copy: fix download from VA surface with odd sizes.

Fix download from Uncacheable Speculative Write Combining (USWC) memory,
e.g. VA surfaces, when the dimensions are odd. In particular, make sure
to get the very last chroma component for each row.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>