lib: Constify igt_buf

No one generally needs to modify the igt_bufs we pass around,
so make them const.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/lib/media_spin.c b/lib/media_spin.c
index f3e68ef..009c780 100644
--- a/lib/media_spin.c
+++ b/lib/media_spin.c
@@ -81,7 +81,7 @@
 
 void
 gen8_media_spinfunc(struct intel_batchbuffer *batch,
-		    struct igt_buf *dst, uint32_t spins)
+		    const struct igt_buf *dst, uint32_t spins)
 {
 	uint32_t curbe_buffer, interface_descriptor;
 	uint32_t batch_end;
@@ -121,7 +121,7 @@
 
 void
 gen9_media_spinfunc(struct intel_batchbuffer *batch,
-		    struct igt_buf *dst, uint32_t spins)
+		    const struct igt_buf *dst, uint32_t spins)
 {
 	uint32_t curbe_buffer, interface_descriptor;
 	uint32_t batch_end;