gallium: Add defines for FXT1 texture compression.

gallium: Add defines for FXT1 texture compression.

This texture compression is exposed by 830 and 915, and to make
MESA_FORMAT match PIPE_FORMAT defines I need a corresponding
PIPE_FORMAT.

v2: Set is_hand_written so we don't try to generate pack/unpack code.

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