drm/mipi-dbi: Remove ->enabled

The atomic helpers try really hard to not lose track of things,
duplicating enabled tracking in the driver is at best confusing.
Double-enabling or disabling is a bug in atomic helpers.

In the fb_dirty function we can just assume that the fb always exists,
simple display pipe helpers guarantee that the crtc is only enabled
together with the output, so we always have a primary plane around.

Now in the update function we need to be a notch more careful, since
that can also get called when the crtc is off. And we don't want to
upload frames when that's the case, so filter that out too.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: David Lechner <david@lechnology.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200612160056.2082681-7-daniel.vetter@ffwll.ch