drm/vram-helpers: Add helpers for prepare_fb() and cleanup_fb()

The new helpers pin and unpin a framebuffer's GEM VRAM objects during
plane updates. This should be sufficient for most drivers' implementation
of prepare_fb() and cleanup_fb().

v2:
	* provide helpers for struct drm_simple_display_pipe_funcs
	* rename plane-helper funcs

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191024081404.6978-2-tzimmermann@suse.de