radeonsi: improve flushed depth texture handling

radeonsi: improve flushed depth texture handling

Use r600_resource_texture::flished_depth_texture for GPU access, and
allocate it in the VRAM. For transfers we'll allocate texture in the GTT
and store it in the r600_transfer::staging.

Improves performance when flushed depth texture is frequently used by the
GPU, e.g. in Lightsmark

[ Ported from r600g commit 37708479608af877986b76302a9c92611d1e23d0 ]

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