drm/fb-helper: Instanciate shadow FB if configured in device's mode_config

drm/fb-helper: Instanciate shadow FB if configured in device's mode_config

Generic framebuffer emulation uses a shadow buffer for framebuffers with
dirty() function. If drivers want to use the shadow FB without such a
function, they can now set prefer_shadow or prefer_shadow_fbdev in their
mode_config structures. The former flag is exported to userspace, the
latter flag is fbdev-only.

v3:
	* only schedule dirty worker if fbdev uses shadow fb
	* test shadow fb settings with boolean operators
	* use bool for struct drm_mode_config.prefer_shadow_fbdev
	* fix documentation comments

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Tested-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/315834/