gallium/radeon: Do not use u_upload_mgr for buffer downloads

gallium/radeon: Do not use u_upload_mgr for buffer downloads

Instead create a staging texture with pipe_buffer_create and
PIPE_USAGE_STAGING.

u_upload_mgr sets the usage of its staging buffer to PIPE_USAGE_STREAM.
But since 150ac07b855b5c5f879bf6ce9ca421ccd1a6c938 CPU -> GPU streaming buffers
are created in VRAM. Therefore the staging texture (in VRAM) does not offer any
performance improvements for buffer downloads.

Signed-off-by: Niels Ole Salscheider <niels_ole@salscheider-online.de>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>