gallium: add pipe_sampler_state::pad member

when zeroing the struct without memset, this needs to be zeroed to avoid
garbage data in the padding

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11886>

src/gallium/include/pipe/p_state.h

@@ -415,6 +415,7 @@ struct pipe_sampler_state
    unsigned seamless_cube_map:1;
    unsigned border_color_is_integer:1;
    unsigned reduction_mode:2;    /**< PIPE_TEX_REDUCTION_x */
+   unsigned pad:5;               /**< take bits from this for new members */
    float lod_bias;               /**< LOD/lambda bias */
    float min_lod, max_lod;       /**< LOD clamp range, after bias */
    union pipe_color_union border_color;