etnaviv: Add support for ETC2 texture compression

etnaviv: Add support for ETC2 texture compression

Add support for ETC2 compressed textures in the etnaviv driver.

One step closer towards GL ES 3 support.

For now, treat SRGB and RGB formats the same. It looks like these are
distinguished using a different bit in sampler state, and not part of
the format, but I have not yet been able to confirm this for sure.

(Only enabled on GC3000+ for now, as the GC2000 ETC2 decoder
implementation is buggy and we don't work around that)

Signed-off-by: Wladimir J. van der Laan <laanwj@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>