drm/bochs: fix framebuffer setup.

The driver doesn't consider framebuffer pitch and offset, leading to a
wrong display in case offset != 0 or pitch != width * bpp.  Fix it.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190627081206.23135-1-kraxel@redhat.com