etnaviv: Clear lbl_usage array correctly

etnaviv: Clear lbl_usage array correctly

Fill the entire array instead of just a quarter. This avoids
crashes with large shaders.
(currently this never causes a problem because shaders larger than 2048/4
instructions are not supported by this driver on any hardware, but it will
cause problems in the future)

Fixes: ec436051899 ("etnaviv: fix shader miscompilation with more than 16 labels")
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Wladimir J. van der Laan <laanwj@gmail.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>