drm: omapdrm: Rename omap_crtc page flip-related fields

The old_fb field is only used to indicate whether a page flip is
pending. Turn it into a bool named flip_pending. Rename event and
page_flip_work to flip_event and flip_work for consistency.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>