drm/vmwgfx: Inline ttm_bo_mmap() into vmwgfx driver

The vmwgfx driver is the only remaining user of ttm_bo_mmap(). Inline
the code. The internal helper ttm_bo_vm_lookup() is now also part of
vmwgfx as vmw_bo_vm_lookup().

v2:
	* replace pr_err() with drm_err() (Zack)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Zack Rusin <zackr@vmware.com>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210525151055.8174-6-tzimmermann@suse.de