drm/vc4: plane: Remove redundant assignment

The vc4_plane_atomic_async_update function assigns twice in a row the
src_h field in the drm_plane_state structure to the same value. Remove
the second one.

Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20210318161328.1471556-2-maxime@cerno.tech