r600g: improve flushed depth texture handling v2

r600g: improve flushed depth texture handling v2

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 (~30%)

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>