drm/i915: Use container_of() in the fbdev code

Use container_of() instead of a cast to get struct intel_fbdev
from struct drm_fb_helper.

Also populate the fb_info->par correctly with the drm_fb_helper pointer
instead of the intel_fbdev pointer.

There's no actual functional change since the drm_fb_helper happens to
be the first member inside intel_fbdev.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>