lib/igt_fb: setup font in igt_get_cairo_ctx

We always want the same boring one, so extract it.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/lib/igt_fb.c b/lib/igt_fb.c
index 4289b50..60b582f 100644
--- a/lib/igt_fb.c
+++ b/lib/igt_fb.c
@@ -382,7 +382,10 @@
 	surface = get_cairo_surface(fd, fb);
 	cr = cairo_create(surface);
 	cairo_surface_destroy(surface);
+	igt_assert(cairo_status(cr) == CAIRO_STATUS_SUCCESS);
 
+	cairo_select_font_face(cr, "Helvetica", CAIRO_FONT_SLANT_NORMAL,
+			       CAIRO_FONT_WEIGHT_NORMAL);
 	igt_assert(cairo_status(cr) == CAIRO_STATUS_SUCCESS);
 
 	return cr;