drivers/intel/gma: fix opregion SCI register for Atom platforms

Most Intel platforms use separate registers for software-based
SMI (0xe0) and SCI (0xe8), but Atom-based platforms use a single
combined register (0xe0) for both. Adjust opregion implementation
to use the correct register for Atom-based platforms.

Test: Boot Windows on Atom-based ChromeOS device with Tianocore
payload and non-VBIOS graphics init; observe Intel display
driver loaded correctly and internal display not blank.
(requires additional change for Atom platforms to select
CONFIG_INTEL_GMA_SWSMISCI)

Change-Id: I636986226ff951dae637dca5bc3ad0e023d94243
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/23696
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
3 files changed