etnaviv: explicitly set nir_variable_mode

No functional changes - fixes the following assert:
  nir_lower_io_impl: Assertion `!(modes & ~supported_modes)' failed.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5997>
1 file changed