efifb: BGRT: Improve efifb_bgrt_sanity_check

For various reasons, at least with x86 EFI firmwares, the xoffset and
yoffset in the BGRT info are not always reliable.

Extensive testing has shown that when the info is correct, the
BGRT image is always exactly centered horizontally (the yoffset variable
is more variable and not always predictable).

This commit simplifies / improves the bgrt_sanity_check to simply
check that the BGRT image is exactly centered horizontally and skips
(re)drawing it when it is not.

This fixes the BGRT image sometimes being drawn in the wrong place.

Cc: stable@vger.kernel.org
Fixes: 88fe4ceb ("efifb: BGRT: Do not copy the boot graphics for non native resolutions")
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Cc: Peter Jones <pjones@redhat.com>,
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190721131918.10115-1-hdegoede@redhat.com