drm: Add drm_gem_vram_{pin/unpin}_reserved() and convert mgag200

drm: Add drm_gem_vram_{pin/unpin}_reserved() and convert mgag200

The new interfaces drm_gem_vram_{pin/unpin}_reserved() are variants of the
GEM VRAM pin/unpin functions that do not reserve the BO during validation.
The mgag200 driver requires this behavior for its cursor handling. The
patch also converts the driver to use the new interfaces.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190516162746.11636-2-tzimmermann@suse.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>