gallium: add a way to query min/max texture gather offsets

Defaults to providing the same offsets as MIN/MAX_TEXEL_OFFSET. For
nvc0, the offset can be -32/31.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
13 files changed