drm/i915: Do not use plane_config in intel_fbdev.c

Use the atomic state instead, this allows removing plane_config
from the crtc after the full hw readout is completed.

The size can be found in the fb, no need for the plane_config.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>