radeonsi: port texture improvements from r600g

radeonsi: port texture improvements from r600g

This started as an attempt to add support for MSAA texture transfers and
MSAA depth-stencil decompression for the DB->CB copy path.
It has gotten a bit out of control, but it's for the greater good.

Some changes do not make much sense, they are there just to make it look
like the other driver.

With a few cosmetic modifications, r600_texture.c can be shared with
a symlink.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>