So, after a lot of trial-and-error, these are the kernel params I’m using:
GRUB_CMDLINE_LINUX_DEFAULT="pcie_aspm=force acpi_osi=Linux acpi_osi='!Windows 2012' i915.preliminary_hw_support=1 i915.i915_enable_rc6=1 i915.i915_enable_fbc=1 i915.lvds_downclock=1 quiet splash"