drm/ast: Add primary plane

Like the original mode-setting code, the primary plane supports XRGB888,
RGB565 and C8. The plane itself only pins BOs and sets the base address
and scanline offset. The mode-setting code will be located in the CRTC's
atomic helpers.

v2:
	* don't set plane call-back functions to NULL explicitly
	* define plane format array in global scope

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107083404.6852-7-tzimmermann@suse.de