kmstest: Fix up lifetimes of cairo objects

cairo_t is the short lived drawing context, whereas cairo_surface_t is
the heavyweight object that persists and is also tied to underlying GEM
objects. So make the kmstest API reflect the different weights and fix
the lifetime and underlying object reference leaks.

Based on the fix by Paulo Zanoni.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

tests/kms_pipe_crc_basic.c

@@ -100,6 +100,7 @@ connector_set_mode(data_t *data, connector_t *connector, drmModeModeInfo *mode)
 	kmstest_paint_color(cr, 0, 0, mode->hdisplay, mode->vdisplay,
 			    0.0, 1.0, 0.0);
 	igt_assert(cairo_status(cr) == 0);
+	cairo_destroy(cr);
 
 #if 0
 	fprintf(stdout, "Using pipe %c, %dx%d\n", pipe_name(config->pipe),