freedreno/a4xx: use hardware RGTC texture samplers

freedreno/a4xx: use hardware RGTC texture samplers

a4xx hardware has real support for RGTC so there's no need to fake it
like we do on a3xx. Undo the hacks, and keep track of an "internal
format" of a resource, which on a3xx will be different, triggering the
transfer-time conversions to take place.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>