drm/ast: Separate DRM driver from PCI code

Putting the DRM driver to the top of the file and the PCI code to the
bottom makes ast_drv.c more readable. While at it, the patch prefixes
file-scope variables with ast_.

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/20200730135206.30239-3-tzimmermann@suse.de