drm/ast: Pin framebuffer BO during dirty update

Another explicit lock operation of a GEM VRAM BO is located in AST's
framebuffer update code. Instead of locking the BO, we pin it to wherever
it is.

v2:
	* update with pin flag of 0

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190613073041.29350-6-tzimmermann@suse.de