drm/cma-helper: Add drm_gem_cma_print_info()

Add drm_gem_cma_print_info() for debugfs printing
struct drm_gem_cma_object specific info.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171107191348.17555-8-noralf@tronnes.org