drm/radeon/kms: rework texture cache flush in r6xx+ blit code

Move the TC flush before the texture setup to match mesa and
the ddx. Also, move the TC flush into the texture setup
function.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>