drm/gud: Map framebuffer BOs with drm_gem_fb_vmap()

Abstract the framebuffer details by mapping its BOs with a call
to drm_gem_fb_vmap(). Unmap with drm_gem_fb_vunmap().

The call to drm_gem_fb_vmap() ensures that all BOs are mapped
correctly. Gud still only supports single-plane formats.

No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210730183511.20080-5-tzimmermann@suse.de