drm: Add |struct drm_gem_vram_object| and helpers

drm: Add |struct drm_gem_vram_object| and helpers

The type |struct drm_gem_vram_object| implements a GEM object for simple
framebuffer devices with dedicated video memory. The BO is either located
in VRAM or system memory.

The implementation has been created from the respective code in ast,
bochs and mgag200. These drivers copy their implementation from each
other; except for the names of several data types. The helpers are
currently build with TTM, but this is considered an implementation
detail and may change in future updates.

v5:
	* do WARN_ON_ONCE for pin-count mismatches
	* allocate only 2 entries in placements array
v4:
	* cleanups from checkpatch.pl
	* removed several fixed-size types from interfaces
	* DRM_VRAM_HELPER now selects DRM_TTM
	* remove separate config option for GEM VRAM
v2:
	* rename to |struct drm_gem_vram_object|
	* move drm_is_gem_ttm() to a later patch in the series
	* add drm_gem_vram_kmap_at()
	* return is_iomem from kmap functions
	* redefine TTM placement flags for public interface
	* documentation fixes

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190508082630.15116-2-tzimmermann@suse.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>