bochs: convert to drm_fb_helper_fbdev_setup/teardown

Currently unloading bochs_drm (after unbinding the vtconsole) results in
a warning about a leaked connector:

    [drm:drm_mode_config_cleanup] *ERROR* connector Virtual-3 leaked!

While investigating a potential fix I noticed that a lot of open-coded
functionality is already implemented elsewhere, so start converting it:
bochs_fbdev_init -> drm_fb_helper_fbdev_setup: trivial (similar impl).
bochs_fbdev_fini -> drm_fb_helper_fbdev_teardown: requires unembedding
"struct drm_framebuffer" from "struct bochs_framebuffer".

Unembedding drm_framebuffer is made easy using drm_gem_fbdev_fb_create
which can replace bochs_fbdev_destroy and custom routines in bochs_mm.c.
For this to work, the GEM object is moved into "drm_framebuffer". After
that, "bochs_framebuffer" is no longer needed and therefore removed.

Remove the unused "size" and "initialized" fields from fb, the latter is
not necessary as drm_fb_helper_fbdev_teardown can be called even if
bochsfb_create fails. This theory was tested by returning early and
late (just before drm_gem_fbdev_fb_create). Both scenarios fail
gracefully although the latter seems to leak the object from
bochsfb_create_object (not a regression).

Guess on the reason for the encoder leak: drm_framebuffer_cleanup was
previously used, but did not destroy much. drm_fb_helper_fbdev_teardown
is now used and calls drm_framebuffer_remove which does a bit more work.

Tested with 'echo 0 > /sys/class/vtconsole/vtcon1/bind; rmmod bochs_drm'
and also with Xorg + fbdev (startx -> xterm). The latter triggered a
warning in ttm_bo_vm_open that existed before, see
https://lkml.kernel.org/r/1464000533-13140-4-git-send-email-mstaudt@suse.de

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Link: http://patchwork.freedesktop.org/patch/msgid/20180906221810.20170-3-peter@lekensteyn.nl
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>