drm/i915: Store the initial framebuffer in initial_plane_config

At the moment we use crtc->base.primary->fb to hold the initial
framebuffer allocation, disregarding if it's valid or not.

This lead to believe we were actually updating the fb at this point, but
it's not true and we haven't even called drm_framebuffer_init() on this
fb.

Instead, let's store the state in struct intel_initial_plane_config
until we know we can reuse that framebuffer.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>