drm/bochs: 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. This
replaces similar functionality in bochs_connector_mode_valid().

v2:
	* remove bochs_connector_mode_valid(), which now serves no purpose

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200203155258.9346-3-tzimmermann@suse.de