drm: Integrate VRAM MM into struct drm_device

There's now a pointer to struct drm_vram_mm stored in struct drm_device.
DRM drivers that use VRAM MM should use this field to refer to their
instance of the data structure. Appropriate helpers are now provided as
well.

Adding struct drm_vram_mm to struct drm_device further avoids wrappers
and boilerplate code in drivers. This patch implements default functions
for callbacks in struct drm_driver and struct file_operations that use
the struct drm_vram_mm stored in struct drm_device. Drivers that need to
provide their own implementations can still do so.

The patch also adds documentation for the VRAM helper library in general.

v5:
	* set .llseek to no_llseek() from DRM_VRAM_MM_FILE_OPERATIONS
v4:
	* cleanups from checkpatch.pl
	* document VRAM helper library

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190508082630.15116-9-tzimmermann@suse.de


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>