drm: Replace drm_gem_vram_push_to_system() with kunmap + unpin

drm: Replace drm_gem_vram_push_to_system() with kunmap + unpin

The push-to-system function forces a buffer out of video RAM. This decision
should rather be made by the memory manager. By replacing the function with
calls to the kunmap and unpin functions, the buffer's memory becomes available,
but the buffer remains in VRAM until it's evicted by a pin operation.

This patch replaces the remaining instances of drm_gem_vram_push_to_system()
in ast and mgag200, and removes the function from DRM.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20190521110831.20200-2-tzimmermann@suse.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>