drm/i915: support 32 bit BGR formats in sprite planes

intel_framebuffer_init does some basic sanity checking of the pixel format,
but is used by the plane code in addition to the primary crtc.  So it
needs to contain any formats used in either place.

Add the XBGR8888 format to the checklist so the plane code can use it.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>