drm/gem: Provide offset-adjusted framebuffer BO mappings

Add an additional argument to drm_gem_fb_vmap() to return each BO's
mapping adjusted by the respective offset. Update all callers.

The newly returned values point to the first byite of the data stored
in the framebuffer BOs. Drivers that access the BO data should use it.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210803125928.27780-2-tzimmermann@suse.de