drm/qxl: add drm_gem_plane_helper_prepare_fb

We could need to wait for the pin to complete here.

Signed-off-by: Christian König <christian.koenig@amd.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Cc: spice-devel@lists.freedesktop.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220429134230.24334-4-christian.koenig@amd.com