efi_loader: Fix GOP 32bpp exposure

We store pixels as BGRA in memory, as can be seen from struct efi_gop_pixel.
So we need to expose the same format to UEFI payloads to actually have them
use the correct colors.

Reported-by: Fabian Vogt <fvogt@suse.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>