gallium: fix various undefined left shifts into sign bit

gallium: fix various undefined left shifts into sign bit

Funnily enough, some of these were turned into a compile-time error by gcc
with -fsanitize=undefined ("initializer is not a constant").

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>