drm/vram: Move VRAM memory manager to GEM VRAM implementation

The separation between GEM VRAM objects and the memory manager is
artificial, as they are only used with each other. Copying both
implementations into the same file is a first step to simplifying
the code.

This patch only moves code without functional changes.

v3:
	* update to use dev->vma_offset_manager
v2:
	* update for debugfs support
	* typos in commit message

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190911110910.30698-2-tzimmermann@suse.de