tgsi: Initialize tgsi_declaration_dimension padding.

Silence this Coverity defect.

Uninitialized scalar variable (UNINIT)
uninit_use: Using uninitialized value dim. Field dim.Padding is uninitialized.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7300>

src/gallium/auxiliary/tgsi/tgsi_build.c

@@ -169,6 +169,7 @@ tgsi_default_declaration_dimension()
    struct tgsi_declaration_dimension dim;
 
    dim.Index2D = 0;
+   dim.Padding = 0;
 
    return dim;
 }