drm/i915/dp: Don't use DPCD backlights that need PWM enable/disable

We haven't yet implemented support for backlights that need to be
enabled/disabled via PWM instead of AUX, which means we'll break things if
we enable DPCD backlight control on these machines. Luckily though since
most of these machines work fine just using the plain PWM backlight
controls anyway, there shouldn't be any issue with just leaving DPCD
backlight controls disabled in such situations.

This should fix the issues with PWM being left on that were being observed
on fi-bdw-samus.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Fixes: 4a8d7990 ("drm/i915/dp: Enable Intel's HDR backlight interface (only SDR for now)")
Testcase: igt/gem_exec_suspend/basic-s0 # fi-bdw-samus
Cc: Lyude Paul <lyude@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210121183644.2627282-1-lyude@redhat.com