drm/bochs: Remove PRIME helpers from driver structure

VRAM PRIME helpers are now called through GEM object functions. The
driver callback functions are obsolete.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190702115012.4418-3-tzimmermann@suse.de