drm/cma: add debugfs helpers

Add helper to display fb's which can be used directly in drm_info_list:

static struct drm_info_list foo_debugfs_list[] = {
		...
		{ "fb",   drm_fb_cma_debugfs_show, 0 },
};

to display information about CMA fb objects, as well as a
drm_gem_cma_describe() which can be used if the driver bothers to keep
a list of CMA GEM objects.

Signed-off-by: Rob Clark <robdclark@gmail.com>