vga: Start cutting out non-32bpp conversion support

Nowadays, we either share a surface with the host, or we create
a 32bpp ARGB console surface.

So we only need to draw/convert to 32bpp, enabling us to remove
all but one instance of vga_template.h inclusion (to be further
cleaned up), rgb_to_pixel_* etc...

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