drm: Move simple_display_pipe prepare_fb helper into gem fb helpers

There's nothing tinydrm specific to this, and there's a few more
copies of the same in various other drivers.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Cc: David Lechner <david@lechnology.com>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Daniel Stone <daniels@collabora.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Acked-by: David Lechner <david@lechnology.com>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180405154449.23038-3-daniel.vetter@ffwll.ch