vga: Separate LE and BE conversion functions

Provide different functions for converting from an LE vs a BE
framebuffer. We cannot rely on the simple cases always being
shared surfaces since cirrus will need to always shadow for
cursor emulation, so we need the full set of functions to
be able to later handle runtime switching.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org&gt;\>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>