drm/gud: Get offset-adjusted mapping from drm_gem_fb_vmap()

Pass the data parameter to drm_gem_fb_vmap() to retrieve pointers
to the data. This address is different from the mapping addresses
for framebuffers with non-zero offsets. Replaces gud's internal
computation.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210803125928.27780-4-tzimmermann@suse.de