r600g: port color buffer format conversion from radeonsi

r600g: port color buffer format conversion from radeonsi

r600_translate_colorformat is rewritten to look like radeonsi.
r600_translate_colorswap is shared with radeonsi.
r600_colorformat_endian_swap is consolidated.

This adds some formats which were missing. Future "plain" formats will
automatically be supported.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>