drm/bochs: Convert bochs driver to VRAM MM

The data structure |struct drm_vram_mm| and its helpers replace bochs'
TTM-based memory manager. It's the same implementation; except for the
type names.

v5:
	* set .llseek via DRM_VRAM_MM_FILE_OPERATIONS
v4:
	* don't select DRM_TTM or DRM_VRAM_MM_HELPER
v3:
	* use drm_gem_vram_mm_funcs
	* convert driver to drm_device-based instance
v2:
	* implement bochs_mmap() with drm_vram_mm_mmap()

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190508082630.15116-14-tzimmermann@suse.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>