drm/pl111: Add optional variant display en/disable callbacks

The silcon and components around the PL111 may require some
variants to perform special set-up of the display. Add two
callbacks to manage this.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170908124709.4758-6-linus.walleij@linaro.org