drm/vboxvideo: Implement struct drm_mode_config_funcs.mode_valid

The implementation of struct drm_mode_config_funcs.mode_valid verifies
that enough video memory is available for a given display mode.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200203155258.9346-5-tzimmermann@suse.de