radeonsi: check ctx->sdma_cs before using it

e5167a92 disabled SDMA for gfx8.
This caused 3 piglit arb_sparse_buffer tests (basic, buffer-data
and commit) to crash on GFX8.

Reported-by: Michel Dänzer <michel@daenzer.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Fixes: e5167a92 ("radeonsi: disable SDMA on gfx8 to fix corruption on RX 580")
(cherry picked from commit 5f8daae4)
Conflicts resolved by Dylan Baker

Conflicts:
	src/gallium/drivers/radeonsi/si_buffer.c