drm/i915: Bad pixel formats can't reach the sprite code

The framebuffer pixel format is already checked by the common code.
So there's no way an invalid format could reach the driver. So instead
of falling back to a default format, call BUG().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>