efifb: Copy the ACPI BGRT boot graphics to the framebuffer

On systems where fbcon is configured for deferred console takeover, the
intend is for the framebuffer to show the boot graphics (e.g a vendor
logo) until some message (e.g. an error) is printed or a graphical
session takes over.

Some firmware relies on the OS to show the boot graphics.

This patch adds support to efifb to show the boot graphics and
automatically enables this when fbcon is configured for deferred
console takeover.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>