mesa: fix _FP32Buffers bitfield use

Previously we were assuming that all color attachments were active.

Fixes: 070a5e5d9248 ("mesa: add explicit enable for EXT_float_blend, and error condition")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35014>
3 files changed