radeonsi: Clear sampler view flags when binding a buffer

Fixes assertion failure while running the Unreal Engine 4 Elemental demo:

.../si_blit.c:322:si_decompress_color_textures: Assertion `tex->cmask.size || tex->fmask.size' failed.

Cc: "10.2 10.3" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/drivers/radeonsi/si_descriptors.c

@@ -422,6 +422,11 @@ static void si_set_sampler_views(struct pipe_context *ctx,
 				si_set_sampler_view(sctx, shader, SI_FMASK_TEX_OFFSET + slot,
 						    NULL, NULL);
 			}
+		} else {
+			samplers->depth_texture_mask &= ~(1 << slot);
+			samplers->compressed_colortex_mask &= ~(1 << slot);
+			si_set_sampler_view(sctx, shader, SI_FMASK_TEX_OFFSET + slot,
+					    NULL, NULL);
 		}
 	}