TGSI's FBFETCH instruction currently only supports reading from a single
render target, but NIR intrinsics can support multiple render targets.

radeonsi can only support fetching from RT 0, but other drivers may be
able to support fetching from any render target.

To express this, this patch renames PIPE_CAP_TGSI_FS_FBFETCH to simply
PIPE_CAP_FBFETCH, and converts it from a boolean "is FBFETCH supported?"
to an integer number of render targets which can be fetched.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>