drm/ast: Fixed CVE for DP501

drm/ast: Fixed CVE for DP501

[ Upstream commit ba4e0339a6a33e2ba341703ce14ae8ca203cb2f1 ]

[Bug][DP501]
If ASPEED P2A (PCI to AHB) bridge is disabled and disallowed for
CVE_2019_6260 item3, and then the monitor's EDID is unable read through
Parade DP501.
The reason is the DP501's FW is mapped to BMC addressing space rather
than Host addressing space.
The resolution is that using "pci_iomap_range()" maps to DP501's FW that
stored on the end of FB (Frame Buffer).
In this case, FrameBuffer reserves the last 2MB used for the image of
DP501.

Signed-off-by: KuoHsiang Chou <kuohsiang_chou@aspeedtech.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421085859.17761-1-kuohsiang_chou@aspeedtech.com
Signed-off-by: Sasha Levin <sashal@kernel.org>