drm/i915: Make ->update_primary_plane infallible

Way back we've used this to reject framebuffers with unsupported
pixel formats. But since the modesetting reorg with the compute
config stage we reject those much earlier and just BUG() in this
callback. So switch to a void return type.

Reviewed-by: Akash Goel <akash.goel@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>