r300: fixes for UB caused by left shifts

r300_vs.c:252:37: runtime error: left shift of negative value -1
r300_state.c:1824:66: runtime error: left shift of 63112 by 16 places
cannot be represented in type 'int'

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13308>
3 files changed