drm/gem: Update internal GEM vmap/vunmap interfaces to use struct dma_buf_map

GEM's vmap and vunmap interfaces now wrap memory pointers in struct
dma_buf_map.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201103093015.1063-8-tzimmermann@suse.de