video: hgafb: fix potential NULL pointer dereference

The return of ioremap if not checked, and can lead to a NULL to be
assigned to hga_vram. Potentially leading to a NULL pointer
dereference.

The fix adds code to deal with this case in the error label and
changes how the hgafb_probe handles the return of hga_card_detect.

Cc: Ferenc Bakonyi <fero@drama.obuda.kando.hu>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@gmail.com>
Link: https://lore.kernel.org/r/20210503115736.2104747-40-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>