drm/i915: Set up fb format modifier for initial plane config

No functional changes yet since intel_framebuffer_init would have
fixed this up for us. But this is prep work to be able to handle new
tiling layouts in the initial plane config code.

Follow-up patches will start to make use of this and switch over to fb
modifiers where needed.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>