drm/i915/dmc: Fix an unlikely NULL pointer deference at probe

In the Linux kernel, the following vulnerability has been resolved:

drm/i915/dmc: Fix an unlikely NULL pointer deference at probe

intel_dmc_update_dc6_allowed_count() oopses when DMC hasn't been

initialized, and dmc is thus NULL.

That would be the case when the call path is

intel_power_domains_init_hw() -> {skl,bxt,icl}_display_core_init() ->

gen9_set_dc_state() -> intel_dmc_update_dc6_allowed_count(), as

intel_power_domains_init_hw() is called *before* intel_dmc_init().

However, gen9_set_dc_state() calls intel_dmc_update_dc6_allowed_count()

conditionally, depending on the current and target DC states. At probe,

the target is disabled, but if DC6 is enabled, the function is called,

and an oops follows. Apparently it's quite unlikely that DC6 is enabled

at probe, as we haven't seen this failure mode before.

It is also strange to have DC6 enabled at boot, since that would require

the DMC firmware (loaded by BIOS); the BIOS loading the DMC firmware and

the driver stopping / reprogramming the firmware is a poorly specified

sequence and as such unlikely an intentional BIOS behaviour. It's more

likely that BIOS is leaving an unintentionally enabled DC6 HW state

behind (without actually loading the required DMC firmware for this).

The tracking of the DC6 allowed counter only works if starting /

stopping the counter depends on the _SW_ DC6 state vs. the current _HW_

DC6 state (since stopping the counter requires the DC5 counter captured

when the counter was started). Thus, using the HW DC6 state is incorrect

and it also leads to the above oops. Fix both issues by using the SW DC6

state for the tracking.

This is v2 of the fix originally sent by Jani, updated based on the

first Link: discussion below.

(cherry picked from commit 2344b93af8eb5da5d496b4e0529d35f0f559eaf0)