drm/omap: add support for manually updated displays

This adds the required infrastructure for manually updated displays,
such as DSI command mode panels. While those panels often support
partial updates we currently always do a full refresh.

The display will be refreshed when something calls the dirty callback,
such as libdrm's drmModeDirtyFB(). This is currently being done at least
by the kernel console and Xorg (with modesetting driver) in their
default configuration. Weston does not implement this and the fbdev
backend does not work (display will not update). Weston's DRM backend
uses double buffering and the page flip will also trigger a display
refresh.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>