i965/blorp: Refactor sRGB encoding/decoding.

i965/blorp: Refactor sRGB encoding/decoding.

Because the rules for sRGB are so insane, we change brw_blorp_miptrees
to take decode_srgb and encode_srgb flags, which control linearization
of the source and destination separately.

This should make it easy to implement whatever crazy combination of
rules people throw at us.  For now, it should be equivalent.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>