drm: omapdrm: Remove omap_crtc enabled field

The field tracks the CRTC state to avoid double-enable or -disable. As
the DRM atomic core guarantees that the CRTC enable and disable
functions won't be called on an already enabled or disabled CRTC, such
tracking isn't needed. Remove the enabled field.

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