drm/ast: Don't use ast->dev if dev is available

Several places in ast use ast->dev, when a dev pointer is already
available within the function. Remove the extra indirection. No
functional changes made.

This is just a small cleanup before embedding the DRM device instance
in struct ast_private.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730135206.30239-6-tzimmermann@suse.de