gallium: dri2: Use index as plane number.

This fix wrong color when playing video under Android + virgl
configuration.

Fixes: 2decad49 ("gallium/dri2: Support images with multiple planes for modifiers")
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Lepton Wu <lepton@chromium.org>
(cherry picked from commit 5a40e153)